Overdubbing is the process of recording new audio on top of previously recorded tracks. It's how most modern music is made: drums first, then bass, then guitars, then vocals — each layer recorded separately while listening to what's already been laid down.
This technique allows each part to be captured with optimal microphone placement, takes, and processing. It also means a single musician can play multiple instruments on the same track.
The opposite approach — recording all musicians playing simultaneously — is called tracking live. Both methods have their place depending on the genre and desired feel.