Expired food is safe to eat as long as it does not contain any bacteria such as bad meat that was left in the refrigerator after 5 days or so.
But if it's meat that is expired and it has remained frozen it's safe to eat well past the expiration date as the freezer keeps bacteria from growing on the meat.
As for other foods such as noodles, chips, cereals, oatmeal and other non meat items they are safe to eat after expiration date as long as no bugs have gotten into the food.
If bugs get into the food that has expired then throw it out but if not it's safe and okay to eat.
The food just gets less tasty past it's expired date and if it's crackers that are expired they will usually taste awful although they are still safe to eat.
If you're in doubt about the expired food being good or bad then just throw it out and don't risk getting sick.