Exactly what snafusmurf said... but let me explain a bit more in case you dont know the terms being used.
1. Rebuys - When a tournament allows rebuys, this means that for a fee, usually the entry fee minus the rake, you can "REBUY" all of your chips back if you should lose them all OR if you get below the starting chip count. Many people like these tourney's, I do not. Its a matter of preference.
2. Add-On's - These tourney's allow 2 "Add-ons". An Add-On also allows you to regain all of your chips for a fee, same as above, but there is usually no requirement. Many people as soon as they get a seat, will buy the first add-on, and during the break take the second add-on if they feel they need it (and can afford it).
Say you are allowed 2 "Add-Ons" in a game, many will take the first add-on as soon as the tourney starts, for 2 reasons: 1) Increase Chip Count and 2) If they double up, well, you understand the math. Usually the 2nd Add-On is taking during the break, the last chance to take advantage of it. Usually there are NO Rebuys or Add-On's after the first break.
Just keep in mind that if you are playing a $1.10 tourney, and take 4 Rebuys and 2 Add-On's, you are now invested $7.10 into the tournament.
Please feel free to add to this post if I left anything out...