Bibek is the practise of only eating from the hands of amritdhari's, people who have been baptised into the Sikh religion.
When I first head about this, I was strongly against it and even ridiculed it saying that it's 'fanatic' or 'extremist'.
The concept is simple, to only eat from the hands of baptised Sikhs who actually practice the religion our Guru's gave us. For example the person must:
1) Follow the Rehit given to us in the baptism/amrit ceremony.
2) Not be bujjar kurehity (have committed any of the 4 sins (Eating meat, taking intoxidants, sex outside of marriage, cutting of hair) which make you peshi ('break your amrit', or mean you have to confess you sin to the 5 pyarey in order to be forgiven)
3) Give Daswandh. Daswandh is one of the most important things in Sikhism, to give one tenth of our earnings to the poor/guru ji. A person who doesnt is considered a thief.
The reason bibeki's follow this is two-fold. The first main reason is because food that is bought from outside, or made by people who dont practice the sikh religion is more likely to be contaminated by meat of some form. I.e Egg's and fish are not considered meat by most vegetarian food places. So even eating at a vegi food place is not considered right.
Also the person who prepares the food, who knows what their level of hygiene is? This is not to say that amritdhari people are instantly cleaner than non-amritdharis but they are expected to wash atleast once a day.
If the person is a smoker the likelihood of the remnant of nicotine being on their clothes is likely. Without washing it is almost impossible to completely remove it once having smoked.
The second aspect of keeping Gurmat Bibek is because the person who makes the food has a direct influence on the 'spiritual nutrition' inside the food.
There are two examples of what is done around the time of food being made, and the outcomes. The first most obvious is the amrit sanchaar ceremony itself. When Amrit is made a sarbloh(iron) bata is used. Into it, is poured water and sugar crystals - it is when TRANSFORMED into amrit.
How is this possible?
The answer is by reciting Bani over it by 5 rehitwaan (people who have kept the rehit) Sikhs. Even if we do not understand the bani fully, the water is made into amrit.
In the same way, if food is made and someone is angry or singing a lustful song then there WILL be impact on the food.
Another example of actions effecting food is when Guru Nanak Dev Ji took two roti's, from one came milk and another came blood.
Blood poured from the rich mans (Malik Bhago) roti who mistreated the poor, working them hard and paying a very meagre wage.
However, milk poured from the roti which had been made by the hard working honest man, Bhai Lalo Ji.
The actions we commit, will have a direct influence on food we eat.
In this way it is likely that someone who has given their head to the Guru, and _should_ have denounced worldly things is more likely to implant the seed of purity into the food - rather than someone who hasnt taken amrit.
The most important thing to do while keeping bibek, is not to get Hankaari (proud) or think that you are better than anyone who does not keep it.
Every amritdhari person should keep bibek, it is a purataan (old school) rehit that has been lost to the Sikh Nation recently but should not be forgotten.
Sarbloh Bibek (is the same as bibek, but only all things are cooked and eaten from iron utensils) should only be kept when a person is doing a certain amount of Kamai. Someone who keeps strict amritvela, and does atleast 2 hours of simran on top of the their nitnem (which may consist of the normal 7 banis and Sukhmani Sahib, aswell as Asa Kee Vaar) for example. This is simply to control the power contained within Sarbloh Bibek, as it is very easy to get proud or think of ourselves as higher than people who dont yet keep the rehit.
Here is an article taken from MKhalsa Blog: Regarding the stages of Sarbloh Bibek...
"Regarding Sarbloh Bibek
I posted this on tapoban today:
Many sarblohis I know, still use water bottles.
Where you draw the line, depends on YOU.
When I started, I still ate bread, chocolate, anything out of a can etc..
But i didnt eat anything that was directly made by a non amritdhari. Only factory made or amritdhari made food.
Slowly, I cut out everything, except for a few things like bread and baked beans, and after awhile, I cut those out too.
For some people it is a long process (for me it was a year or two, and I still do not eat from only sarblohi bibekis, but rather all amritdharis, so I am still in the process).
It depends on you and where you are. My advice is do it, but dont do it slowly, dont be lazy about it. Do it at a pace you are comfortable with though.
Process suggestions for the real beginner (just find out where you are on this):
1. Stop eating meat, or drinking alcohol, stay away from tobacco.
2. Stop eating at places that serve meat or where meat can contaminate your food. (or alcohol).
3. Stop eating out period.
4a. Stop eating from monays, not necessarily from family though (unless they are radhaswami or some other narakdhari)
4b. Stop eating from all non-amritdharis, including family
5. Stop eating really procesed foods/junk foods. For example, chips (crisps), baked beans, ketchup etc.. just stuff thats been in a factory for years or in cans for years, its gross, give it up.
6. Stop eating at gurudwara, *unless they make langar with full rehat that you are now following* (here is where it really starts getting hard). I wont go into the details of why. I am just giving an overview.
7. Stop eating all processed foods, except for a few things, like tofu, bread, cheese, basic things.
8. Start eating and cooking in sarbloh. If its not made in sarbloh, dont worry, just eat it in sarbloh.
9. Cut out all outside foods that you are no longer comfortable with. ie, maybe you feel comfortable with butter, cheese, tofu etc. Still drink juice perhaps?
10. Eat food only cooked in Sarbloh
11. Eat food only cooked in sarbloh, by sarbloh bibekis and cut out unecessary things like cheese, butter, juices, tofu etc. You can make it at home for nothing!
Hope this helps. This is just my view of it...."
The reason why food brought from shops that has been cooked cannot be eaten (such as bread etc) is because it HAS been cooked by someone non-amritdhari. It all depends how far you are practically able to take it.
Some Bibeki's do not eat from the gurudwara either, as the people making the food may not be amritdhari. Food does not become langar because we put it before Guru Ji. If it did the panj pyarai wouldnt have to keep rehit to make amrit, anyone could do it aslong as they knew the 5 banis.
Bibek is a tool to help us get to God.
Many of the the latter day Bhagats/Sants have kept this rehit, for example Bhai Jeevan Singh Ji, Bhai Rama Singh ji, Sant Jarnail Singh Jee Bhindranwale, Bhai Randhir Singh Jee.