To raise is to increase the size of the bet required to stay in the pot, forcing all subsequent players to call the new amount. If the current bet amount is nothing, this action is considered the opening bet. A player making the second (not counting the open) or subsequent raise of a betting round is said to re-raise.
Standard poker rules require that raises must be at least equal to the amount of the previous bet or raise. For example, if an opponent bets $5, a player may raise by another $5 (or more), but he may not raise by only $2. The primary purpose of the minimum raise rule is to avoid game delays caused by "nuisance" raises (small raises of large bets, such as an extra $1 over a current bet of $50, that have little effect on the action but take time as all others must call). This rule is overridden by table stakes rules, so that a player may in fact raise a $5 bet by $2 if that $2 is his entire remaining stake.
In most casinos, fixed-limit and spread-limit games cap the total number of raises allowed in a single betting round (typically three or four, not including the opening bet of a round). For example in a casino with a three-raise rule, if one player opens the betting for $5, the next raises by $5 making it $10, a third player raises another $5, and a fourth player raises $5 again making the current bet $20, the betting is said to be capped at that point, and no further raises beyond the $20 level will be allowed on that round. It is common to suspend this rule when there are only two players betting in the round (called being heads-up), since either player can call the last raise if they wish. Pot-limit and no-limit games do not have a limit on the number of raises.
If, because of opening or raising, there is an amount bet that the player in-turn has not paid, the player must at least match that amount, or must fold; the player cannot pass or call a lesser amount.