You restart the game, except for 3 things: 1) you get to go first if you haven't played your land that turn, and 2) life totals don't change, and 3) you get to float extra mana to cast additional spells.
This means on turn, say, 7, you could cast Upheaval, float a mana, play a land and then cast Stoneforge Mystic and go get something absurd. You could on 9 lands cast Upheaval, float 3, and then play a land and cast Jace. In fact, any threat that can win the game on its own decently quickly is a huge play after an Upheaval. And you get to take advantage of extra mana while your opponent starts on turn 1. It can also randomly force them to discard a ton of useful cards, which is not of huge relevance all the time but can be useful in Cube, a format where you only have 1 of any particular card.
And, at its worst, you just get to start over and clear your opponent's board if you need some breathing room. The effect is very, very powerful.