Guarantee
A guarantee is a guaranteed prize pool for a tournament, set by the poker room that hosts the event. If not enough players enter, the room pays the difference.
The simple explanation
A tournament may say that the prize pool is $50,000 guaranteed. The room promises that amount even if fewer players come.
A closer look
Guarantees attract players. If the entries do not build the guaranteed prize pool, the room must add the rest, which creates an overlay. For example, with a $500 buy-in and a $50,000 guarantee, the room needs 100 entries to reach the guarantee on buy-ins alone. An overlay is good for the players, because the prize pool is larger than what they paid.
A practical example
A tournament has a $50,000 guarantee, but only 80 players enter at $500, so the room adds the missing $10,000.
Full explanation
A guarantee is a guaranteed prize pool for a tournament, set by the poker room that hosts the event. If not enough players enter, the room pays the difference.
A guarantee promises a minimum prize pool.
If entries are too few, the room adds the difference.
The difference is called an overlay.
An overlay is good for the players.