A player start is something one must place at some point in any map in order to play it in single player. There are four standard types of player starts, which provide locations at which three other players may spawn, and then four more specifically for use in ZDoom, at which four more players, allowing up to eight total, may spawn.
There is one special consideration for player start spots, however, that makes them unlike any other placeable weapon, item, decoration, monster, etc. Without a special MAPINFO entry, FILTERSTARTS, player starts can't be made to appear only in certain skill levels. Without putting FILTERSTARTS in your MAPINFO, you won't be able to adjust where the player starts out the level based on the skill level that is chosen. FILTERSTARTS allows that, letting you accomplish things like starting a player off after an ammo cache with a one-way door keeping them from it, or letting a player start off already past a difficult puzzle.