General

They don't *expire*, but they do taste less good.

Dates on food products are sometimes "Sell By" dates, which means the product should be sold from the store by that date. They're also called "Freshness Dates", meaning the product will not be as "fresh" if sold after that date that date. Likewise "Best Used By" dates mean that after the date listed the taste, or texture, or other characteristics of the product may diminish if you use them after that date.

They're NOT the same as "Expiration Dates" on products like medicines which mean the product should not be used at all beyond that date.

The dates on food products don't mean the product is somehow less safe after the date, but that the customer shouldn't expect the product to be as "good", tasty or "fresh" as the same product used before the date.

In the case of instant mashed potatoes, it's probably fine to go ahead and use them if you're hungry. It's not likely you'll notice any difference. Just be sure they look OK when you pour out the flakes and aren't moldy or anything like that.