A casino host typically determines how much comp (complimentary) you are eligible for based on your level of play and the amount of money you have spent at the casino. This can include factors such as the frequency of your visits, the duration of your play, the amount of money you wager, and the games you play.
free play is just what is sounds, chances to play for given to you by the casino for no cost. You can get them through promotions or for signing up for their players club. Those are usually the cheaperR$5-$10 ones.