I buy 20% if a company for 1 million dollars. Now the evaluation of the company is 5 million.

The company goes public. It is now selling great in the market and the total evaluation is now 50 million.

If said investor sells all of his stock (20%) to people that want to buy it he now has 10 million. So he made 9 million total.

This is obviously not taking into account things like how him selling all his stock effect price, but we need to make it simple.