A deathmatch start is a special kind of player start. It determines where players will spawn in a Deathmatch game, both at the beginning of a match and upon being fragged. The deathmatch start at which a player spawns is normally chosen randomly, and it is best to include more to make the match less predictable and hence less susceptible to being ruined by spawn-camping.
ZDoom includes an optional dmflag, "Spawn Farthest", which forces the respawn code to attempt to find and use the deathmatch start furthest away from all other players when respawning a player. In theory, this gives the respawning player more of a chance to get his bearings and locate a weapon before the other players find and attempt to kill him again.