To use one, you need a letter of introduction,from your GP ,Hospital, Social services, CAB.
Once you have the letter you can use the food bank 3 times , that's in my town,it varies from place to place.
As said, they are an emergency measure , not a permanent solution. A lot of people seem to think you can just roll up and get your shopping free. They are all regulated with restrictions on how often you can use them.