Definition of rebuy
Rebuying can mean two different actions depending on whether you are playing tournament poker or cash game poker. The rebuy in tournament poker is the action of buying back a stack when you are out of chips and have lost all your chips. If the tournament is a rebuy, it may also allow you to add chips to your stack (up to a maximum of 2 full racks). When in cash games, the action of "rebuying" means adding money to your stack. This can be a rebuy from 1 up to 100 big blinds. Generally, cash games limit the number of big blinds to 100.
If the tournament is rebuy, read the details carefully
Rebuy tournaments are very common. But there are tons of different types of rebuy tournaments. For this reason, we encourage you to read the tournament details carefully. Some tournaments allow only 1 rebuy. Some tournaments allow unlimited rebuys in the first hour. Some tournaments allow rebuys for the first 2 hours. Some tournaments allow a rebuy even when you have chips left, others do not; you should ask around.