Mean reversion (finance)

Mean reversion (in finance) is like a see-saw. A see-saw has two sides, and when it is balanced, it means both sides are the same height. In finance, when one side (like a stock price) goes up too high, it means that it is out of balance and it will eventually go back down. Like the see-saw, over time the stock price will eventually go back to what it normally is or the mean. That's why finance people say it “reverts back to the mean” or mean reverts.