Definition of raiser
The raiser is the player who raised preflop. If you simply paid your blind preflop, you will be the limper. If you made a raise, you will be the raiser. The raiser is said to have the initiative. The official raiser of the hand will lose initiative if another player over-raises it. For example, on a €cash games to €1/2 game table, a player raises to €6. A player a little further down the table over-raises (3bet) to $20. Now the player who raised the €20 raise will have the initiative.
The advantage of the raiser
The player who is 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 down the line calls the raise, it is assumed that the player will not have a premium in his range, otherwise he would have over-raised. The advantage of the raiser is that even if he didn't hit the flop, he will still be able to make a cbet to represent a lot of strength. Even if he missed his flop, his opponents will have to give him a lot of credit. The raiser is the one who takes control of the hand.