Comp is an abbreviation of complimentary. Casinos provide services, accommodation, food, show tickets, and huge discounts for airline tickets to attract players to gamble at their casino.
Comps can be achieved by all players and not only high rollers. Best thing to do is simply to ask for it after you have played a while.
Determine your Comp Eligibility
Casinos set your comp eligibility like this: average bet x decisions per hour x hours played x house advantage.
Example: Blackjack - $20 per hand - 4 hours played - 70 hands per hour - house edge is 2%: 20 x 4 x 70 x 2 / 100 = $112. A casino usually returns 40 % in comps. In this example they will give you a comp for $45 or a dinner for two.
Go to the guest service desk or cashier and ask for a VIP, slot club or player's club card. Use this card each time you play and get a card from every casino you visit.
While using it you are being rated. The casino staff will estimate you based on the amount you bet, which games you play, and the amount of time you spend on each game.
Online casinos offer comps as well.