1. Literally, to be past the date by which an edible item should be sold before it spoils. That milk is definitely past the sell-by date—it's curdled!
2. By extension, to be past the peak of one's abilities or career. The company is forcing my mother to retire because they think she's past her sell-by date.That player is definitely past his sell-by date—he hasn't had a good season in years.