transfer purchases or erase and sync?

ok so my computer decided to crash the other day and i lost my itunes of course. i do have some songs backed up but some i didnt. well forturnatly when i plugged my ipod in i was able to get back all the purchases and just not my albums that i added which is fine i'm loading them back in gradually. my question is: i went to buy new song on itunes and when i went to sync the song to my ipod and box saying "erase and sync" popped up i'm not sure what that means and what's going to happen if i do that. i really would appericate if someone could help me out of this mess. thanks

Message was edited by: jamendtea