Answer: Frontline players.
Explanation: In order, it's the spiker, the frontline players, the server, and the back line players. The backline players are obviously in the back, and the spiker needs to be in the way front in order to spike it down over the net. The secondary line of defense are the frontline players, who are obviously in the front, hence the name, although not as far front as the spiker. After that is the server, who needs to be nearer to the back, but not all the way in the back.