Many casinos keep track of their players activities and rate them accordingly. This they do primarily to allocate them a comp level and then offer them comps (complimentary credits, bonuses, freebies) as a reward (see Casino comps guide on this site). This makes the player feel important and valued and more likely to come back.
The player rating system starts at the playing area. It is the supervisor/floorman who keeps track of the players activity. He writes down how much a player buys in for, what is his average bet, what is his highest bet, how fast he plays, how long he plays and how much he wins or loses, and maybe other additional factors.
When the player leaves the table the data is handed in to the data room and all the information is entered on the computer. This activity tracking is done each time the player plays and is added to the data already stored on computer.
Then, when a player uses the facilities of the casino like eating dinner at the restaurant or a snack at the bar or ordering cigarettes, the manager or the pit boss can look up his play history and decide whether or not a comp is justified.
Depending on your rating level and the casino that you play in, you could qualify from a free meal to free hotel room. If you are a high roller, you could qualify for airfare as well.