You can also view previously answered questions in the HSW Question Archive . The package is filled with a mix of gases that are beneficial to the food.
Most foods do better if they are stored away from oxygen.
The amount of time MAP can add to shelf life can be amazing. That's not bad when you consider that the cost of adding the gas is practically zero. It can also extend the seasons of certain fruits and vegetables. The green leaves are still alive even after you pick the lettuce, and you want to keep them alive as long as possible by changing their behavior. It discourages bacteria.