A poker tournament with a guaranteed prize pool (GTD) is when the the prize pool has been set to a minimum. Most online poker sites give this incentive because it tends to attract more players.
The “overlay” is the gap that exists when the guaranteed prize pool doesn’t have enough player buy-ins to cover the guaranteed amount, so if there is an overlay it is paid by poker room.
GTD tournaments with huge overlays provide a lot of value because with fewer entrants competing for the same guaranteed prize pool, the players who are participating in the tournament have a better chance of winning or making it in the money.
Many guaranteed tournaments have re-buys and add-ons available for the first hour which allows players to re-buy back into a tournament if they happen to lose all of their chips.