When you’re participating in a poker tournament, the platform or poker room that hosts it can set a guarantee on a prize.
If not enough players sign up to meet that prize amount, the organizer will add the additional money to meet the guarantee they’ve previously set. This is what it’s known as an overlay in poker.
For example, let’s say that the poker tournament you’re participating in has a guaranteed prize pool of $100,000. The entry fee is $500, and this amount goes directly into the prize pool.
But, only 180 players sign up for the tournament, contributing a total of $90,000. As the poker room promised a $100,000 prize, it is obliged to add the missing $10,000 into the prize pool.