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.