Non-disclosure agreements (or NDAs, if you're fancy) are a lot like pinky-promises. Except, if you break an NDA, you're going to end up with a lot more than an angry friend. In fact, you might end up at the wrong end of a lawsuit.
They're put in place for a variety of reasons, but many have to do with keeping industry secrets from falling into competitor hands.
Some NDAs expire after a few months, years, or even decades. Others may be indefinite, or as long as a secret is meant to stay secret. But many end at some point, leaving those no longer under their grasp to tell their secrets. These Redditors did just that, actually.