It always depends on the product. Usually it's "best before" meaning until this day the manufacturer guarantees for quality / taste / safety, sometimes there's additional information like it has to be stored below a certain temperature or so. This means afterwards it might not be tasty but still save, or it can be unsafe. It's best to trust your own sense and mind - how does it look like? Does it taste strange? How is the smell? Then better be safe than sorry.

There is no general rule as it depends on the product - something which needs to be cooled will have a shorter time (like fresh milk), others will last nearly forever (canned soups)

However, often enough there is a "use by" date which means it has to be eaten before a given time as it will become unsafe afterwards (eggs, ground meat)