This is a term used in poker when a player wants to increase the amount that has been previously bet. "Raise" can be used as a noun e.g. "He put in a $10 raise" or a as verb e.g. "He will raise the pot".
To avoid ambiguity most casinos request you verbally announce your raise by saying "raise" prior to placing any chips into the pot. There is also typically a 3 raise limit to prevent a player from being trapped in the middle and cheated. In Mississippi 4 raises are allowed. Colorado has a 5 raise limit, mainly because state law only allows $5 maximum bets and it's the only way for players to build big pots
Despite what it commonly seen in movies and TV, saying "I call your bet, and raise you" does not constitute a valid raise. Once "call" is said the bet stands as a call. This prevents you from waiting to say "raise" until after you see how your opponents react when you say call. Also not allowed (for the same reason) is putting out the amount of chips for a call and then going back to your stacks of chips and putting in a raise amount. Your call and raise chips must go into the pot together or you must verbally announce your raise. Doing this the wrong way is called making a string bet or "string betting".