The meaning of an efficiency apartment is an apartment that incorporates the bedroom, living room, kitchen etc all into one room.
It uses a small space to complete the apartment so your efficiency apartment would have the bed, the kitchen, the living room, the dining room all in one space.
Although your bathroom of course would be in a separate room than the other rooms.
That is why the efficiency apartment is called the efficiency apartment because it's efficient to incorporate the bed, kitchen, living room, dining room etc all into one space.
That allows more apartments to be built in the same building.
An efficiency apartment is like a hotel or motel room with everything in one space.