The term freeroll in poker is used to describe a situation where a player does not have to pay to play or a situation in which there's no more downside risk involved. It's an opportunity to play at no cost, but yet with the possibility of being rewarded.
You have many choices to consider when looking at what poker site has the best freerolls. Some of the top choices include places like PokerStars, 888poker, GGPoker, and PartyPoker. You can look around online to see what games are open and what you might be interested in playing the most.