A "studio apartment" is an apartment that just has one main room. The bathroom is separate, and the kitchen might be too. But the residents sleep and live in one room.

This is different from a "one bedroom", which has a separate bedroom and living room.

When you're talking about the subject of apartments or housing, you can just call it "a studio". You don't have to call it "a studio apartment". But "a studio" can mean other things as well, such as an artist's studio or a music recording studio.