A spawn spot is a target for spawn cubes ejected by the monster spawner, and is part of the final boss. The spawn spots are things defined in the source code, but they are rendered invisibly by the engine, and thus are not directly visible (they have no sprite associated to them, in any case).
The monster spawner fires its cubes toward each spawn spot in rotation, in the order they appear in the THINGS lump. When a cube reaches a spawn spot, a burst of yellow flames is rendered, and a hellish monster (excluding the two major bosses) appears in the location. If the spawner and its spawn spots are placed in MAP30 any cube landing at a spawn spot currently occupied by a player or monster will telefrag it. If the boss-related things are placed in another level, however, the new monster and the thing currently occupying its location will simply become stuck together.
There may be up to 32 spawn spots in a level, and if there are no spawn spots, a spawn shooter will not create any cubes. If there are too many spawn spots, the game will crash immediately after the spawn shooter awakens. This is due to overflowing the array used to store pointers to the spots.