Notations of “Sell by,” “Use by,” or “Expires on” protect the seller and remind the buyer when the stuff was purchased.
Common sense dictates that meat and seafood should be used quickly for best flavor and healthfulness. If you can’t use it soon after purchase, freeze it.
Dairy products may “go off” before the expiration date. It all depends on how they were handled before you get them home.
Rely on your eyes and nose. If it looks bad or smells bad, it is!