It means that there should be some specific entity that you are directly controlling, e.g. a single adventurer going into the Dungeon of Doom, like in Rogue. This entity could consist of multiple bodies, e.g. A tight squad of marines fighting on a desolate spaceship (Steam Marines) or be leading a bunch of autonomous minions around, e.g. The summoner with throngs of disposal demons (Demon).
Basically the game you should be designing should be fairly character driven, something the player can immerse themselves in. You might tell the story of the Starship 5NX-117, aka, the experimental vessel escaping into the stars. Trying to tell a story about a Star Flotilla with many disposal ships won't really work.