It was created on April 1 for April Fools' Day. There is a timer that counts down from 60 seconds, and pressing the button resets the timer. The button can only be pressed only one time by a Reddit account that was created before April 1, 2015.
Nearly 650,000 users and counting have pressed the button.
When the timer hits 0..... nobody knows what happens.
It's driving people crazy.
It can be hard to explain what (or why) the button is to a non-Redditor, but Reddit user Karlio posted a huge list of everything about the button, and you can go down that rabbit hole as far as you want to. It goes deep.