"Sell by" means the store should sell the product by the printed date; you can still eat the product safely after that date.
"Best if used by" means you should use the product by the date listed for best quality and flavor (not for safety reasons).
"Use by" is the last date recommended for use at peak quality. After that, there will be a noticeable deterioration in flavor, appearance, and texture.
With the exception of infant formula, if the date passes during home storage, it should still be safe to consume the product if it's handled properly until the time spoilage is evident.