If a player cannot bet the full amount it takes to call (or make the blind or straddle bet), they can bet all their remaining cash. The player is then tapped out. The player keeps playing but makes no more bets (and, in particular, cannot raise).
Most poker games have a minimum and maximum buy-in amount. AR$2/$3 No-Limit Hold'em cash game might feature a $40 minimum andR$300 maximum buy-in. At any time during the game, you can top up and replenish your stack back up to the maximum buy-in.