I used to be one of those people who panicked when I saw that the date stamped on my yogurt, milk, or eggs had passed. I was blind to whether it said "sell by" or "use by": if the date had passed it went straight to the trash, no questions asked. It turns out I probably wasted a whole lot of money and ingredients by doing that.
This label is an indicator for retailers: stores should sell the product by this date in order to ensure it will be fresh for customers. Think of it as the last date the item will be at its highest level of freshness; in other words, the item will still be edible after that date.