Freeroll refers to a free-to-enter tournament with actual prizes, meaning you can win some money without risking anything in such an event.
Freerolls can be a good way to learn the basic poker rules if you are just starting, but a relatively small prize pool and big time commitment do not make it worth playing after that stage. Of course, if you are starting from scratch and do not have any bankroll yet, it can help you build it at the beginning.
Apart from building the initial roll, you won’t learn much in these tournaments because of the low level of competition and players just moving all in left and right. As soon as you have some money, moving to real money games and learning there is a much better option if you want to be playing poker for the long run.