To raise in poker is to make the amount of one's bet greater than the amount of the immediately preceding bet, forcing all subsequent players to Call the new amount. If the current bet amount is nothing, this action is called simply a "bet" or Open.
A player making the second (not counting the open) or subsequent raise of a betting round is often said to reraise.
A universal rule in American casinos, and common in home games as well, is that any raise must at least equal the amount of the previous raise. For example, if a player in a Spread limit or No limit game bets $5, the next player may raise by another $5 or more, but he may not raise by only $2, even if that would otherwise conform to the game's Betting structure. The primary purpose of this rule is to avoid game delays caused by "nuisance" raises (small raises of large bets that don't affect the bet amount much but that take time). This rule is often 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 many casinos, there is a limit to 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 head up).
