a freeroll is a tournament where you can possibly win without risking anything. in a tournament players will pay a buyin and all buyins will be pooled. then they will all play until only one player remains. the top N remaining players will get a share of the prize pool. in a guaranteed tournament, the prize pool is guaranteed a certain amount even if less players will play. just like the 10K Welcome Tournament, for 1.10 chip buyin, the prize pool will be guaranteed 10,000 chips (1 bitcoin). So the site will stand to lose if less players play. this is an attractive scenario for players since the value of the seat is a lot more than what they paid for.