What is a Freeroll? well I can put is simple for u if I can. Ity is like any other multi table tournament except it is free. You can improve your skill if you join one online that offers daily freerolls. Once you know the game well then play in one for cash prize at the game tables. It is simply a tournament.
Also, there are certain "freerolls" that you can actually win cash or points that can be used to play in "buy-in" tournaments at most online poker sites. Look for them under tournament tabs at most online poker rooms.