Definition of raiser
The raiser is the player who raised preflop. If you simply paid your blind preflop, you are the limper. If you made a raise, you are the raiser. The raiser is said to have the initiative. The official raiser of the hand will lose the initiative if another player raises. For example, at a 1:2 cash game table, a player raises to 6. A player further down the table raises him (3bet) to 20???. The player who made the raise to 20??? will now have the initiative.
The advantage of the raiser
The player who is the raiser has an advantage over the other players. He is said to have the initiative because he has shown the most strength so far. If a player raises preflop for example, he can have any premium in his range (AA, KK, QQ, AKs, etc). If a player a little further away calls the raise, we assume that he will not have a premium in his range or he would have over-raised. The advantage of raiser is that even if he didn't hit the flop, he can still make a cbet to represent a lot of strength. Even if he missed the flop, his opponents will have to give him a lot of credit. The raiser is the one who takes control of the hand.