An Extension is a technical note placed directly to the right of the Character name that denotes HOW the character's voice will be heard by the audience. An Off-Screen voice can be heard from a character out of the camera range, or from another room altogether.
Frankie pulls all the covers off of Julie. She sits up in bed, pulls on a long T-shirt, then swings her legs onto the floor and shuffles off to the bathroom. FRANKIE (continuing) You're welcome. (beat) Hey, how long you gonna be? I've got a meeting and I need to shower. JULIE (O.S.) Twenty minutes.
Some writers use O.C. (off camera) in place of O.S. The "beat" used above simply denotes that Frankie pauses (perhaps formulating his next thought) before uttering his next bit of dialogue.
Another common extension is V.O. That stands for Voice Over. Think of a V.O. as a narration, or a character speaking while s/he isn't in the scene. Or s/he can be in the scene, but also acting as narrator, reflecting on and describing some time gone by. This dialogue is recorded and then laid in over the scene in editing.
FRANKIE (V.O.) I knew I wasn't gonna get in that shower for at least 45 minutes, so I went for a run.
Our character Frankie is reminiscing about the morning on the boat in a Voice Over.