MRE stands for "Meal, Ready-to-Eat." An MRE comes in a plastic pouch called retort packaging. The package is actually aluminum foil and plastic layered together.
An MRE pouch contains normal food. It might contain chili or beef stew. The food is pre-cooked, put into the pouch and the pouch is sealed. The pouch and the food it contains are then heated for sterilization purposes.
Since the pouch is sealed, the food stays fresh for a long time. This is no different from metal or glass canning -- the food in a can is sterilized by heat in a sealed container so bacteria cannot get in and spoil it. Think of retort packaging as a flexible can. The food tastes no different from canned chili or beef stew.
Retort packaging is also used for a variety of drinks. Those drinks that come in a flexible pouch are usually made using the same technology used to create MREs.
For more information on MREs check out the links below.