Comps are complimentary items and services that casinos offer to players as sort of a loyalty bonus; not necessarily to thank you for playing so far, but to keep you playing more.
Casinos used to give comps only to big-time players ("high rollers") but now you can get them much easier - in fact, play any slot machine a live casino for an hour or two and you'll almost surely get free drinks and food.
Slot machine players lose the most money but also get the most comps; for example, a $1 slot machine player can expect more comps than a $10 blackjack player.
The most effective advice to getting comps as a slots player is to ask for them. (Works in any situation in life, by the way.)
Getting comps is not a good reason on its own to play casino games; you'll still lose money to the casinos. They're just giving a portion of it back.
To increase your chances of getting comps, always sign-up for a slots club membership (also known as "Player's Club"). You'll get comps even if you're not much of a gambler, probably just for signing-up.
By joining a slot club, you'll be "comped" for your play much more likely since they'll be able to track how much you've wagered. (In most cases, you get comps based on how much you wager, not how much you win or lose.)
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