Guarantee (GTD)
Also called guaranteed prize pool
A Guarantee (GTD) is a tournament's promised minimum prize pool, paid out in full even if the buy-ins collected don't add up to that amount.
In a tournament with a guarantee, the host promises that the total prize pool will be at least a set number, no matter how many players show up. You'll often see it written right in the name, like a "$10K GTD," which tells you the winners will share at least that much in real prize money.
It comes up when you're picking which tournament to enter. If enough players register, their combined buy-ins cover the guarantee and everything is funded by the entries. If too few register, the buy-ins fall short of the promised number and the host has to add the difference out of its own pocket. That added money is what players call an 'overlay.'
For a beginner, the takeaway is simple: a guarantee tells you the floor for the prize pool, not the ceiling, so a popular event can blow well past its GTD. An overlay is genuinely good for players, since you end up competing for more money than the entries actually contributed, but it has nothing to do with how you play a given hand.
Example
You sit down in a $20 buy-in, $5,000 GTD online tournament, but only 180 players enter, so the buy-ins add up to $3,600. The host adds the missing $1,400 to reach the guarantee, so that $1,400 is the overlay, and the field is playing for more than the entries actually contributed.