Probably nothing.

Sell-by dates are often there to make sure the product moves off the shelves at a certain rate.

Product manufacturers put the date on, which makes it awkward for the retailer to keep it on the shelves. Retailer moves the product at the prescribed rate, whether sold or tossed in the garbage, manufacturer wins.

It overwhelmingly has little or nothing to do in reality with the quality of the product. Just use your senses instead: Is the package pressurized and bulging? Is it opened when it should be sealed? Is the food smelly in a bad way? Is something growing on/in it that isn't supposed to be?

The date means capitalism, it means nothing to do with the quality or freshness of the product, truthfully. It is at best a hint or guideline. At worst, it causes tremendous waste and leads to throwing away perfectly serviceable goods in the trash.