In a regular poker tournament, if a player wants to win the pool prize, the player must never run out of chips. Once the chips are gone, the player is dropped from the tournament.
Not the case in re-buy poker tournaments. Re-buy poker tournaments allow you to purchase chips again when you run out of them. As such, a player has the opportunity to retain one's seat and play longer. This gives the player a chance for a comeback control of the table.
Nonetheless, there are rules governing re-buy poker tournaments. In short, rebuy poker tournaments are not completely anarchic when it comes to chip-purchasing.
A re-buy tournament has a re-buy period which, usually, is set at an hour prior to the first break. After the set re-buy period, players can no longer make subsequent purchases.
In some cases, players are allowed to make as many re-purchases as they want. In some cases, players are permitted to have only a defined number of re-purchases.
In re-buy poker tournaments, add-ons are allowed. Add-ons are chips you can buy to add more stacks to your chips. In short, an add-on is a subsequent re-purchase of chips one can make regardless of whether a player still has stacks of chips or none. Buying of add-ons are generally done on the first break. Buying of add-ons is limited. Generally, you can get an add-on only once.
Costs of re-buys are not the same for all tournaments. Most of the time, however, poker rooms set re-buys at the same price as the original buy-in. Players are not required to pay the rake again, only the buy-in. Say, you join a $20+2 re-buy tournament, you only give twenty dollars to the house. Usually, you will receive the same chip count as you did with your original buy-in.
Add-ons, generally, can be bought at the same price as the original buy-in. You may receive an equal number of chips as with the original buy-in, or you may receive more.
Subsequent buy-ins and add-ons eventually are added to the prize pool. As such, the total size of the prize pool can only be determined once the period for re-buys has ended.
In a nutshell, re-buy poker tournaments create more opportunity for you. One, they allow you to play poker longer. Unlike other poker tournaments, re-buy tournaments allow you to make subsequent purchases once you run out of chips so you don't get eliminated from the game soon. Two, re-buy tournaments allow you to win bigger cash prize since the more re-buys there are, the bigger the prize pool becomes.