Basicly scientists from around the world take into account the enviroment, wars, nuclear armaments, and world politics crunch some numbers and guesstimate how close we are to end of the world at any given moment.
So as of right now at any given moment we are potentially 90 seconds away from us obliterating ourself ( doesn’t always have to be nukes, could be an environmental catastrophe)
Listen to Iron Maiden’s “2 Minutes to Midnight” to get an idea