Wireneck is correct. Overdubbing is a specific process used before multi-track recorders became available. It involved layering one part of a song on top of another part until reaching the final product. Guitarist Les Paul was a pioneer of this process until 1957 when he went eight track.
The basic process involved two tape recorders. A click track such as a metronome, drum part, rhythm guitar or other tempo basis was recorded on the first tape machine. Another song part was then played as the first tape was played back, causing both parts to be mixed and recorded on the second tape recorder. The process was again reversed and it went back and forth until the end of the song was reached.
I recorded this way for twenty years and it is a skill to be acquired. Nowadays, mixing and processing decisions can be individually made once all tracks are side by side. But with overdubbing, decisions about the level mix, EQ and other processing issues of both the playback and the live playing in the next pass had to be made each time. One could wind up with a final product where the bass was too loud, the vocal was too weak, etc., and it was too late to fix it. A lot of tries were involved each time; sometimes doing a song over and over again until the level balances, frequencies and overall quality were what you wanted (earlier generations became weaker and more distorted as more levels were added.
I'm pleased to have gone through that era and had learned how to do it. It also helped that Les and I were friends at the time!
Incidentally, when folks nowadays add a track to a multi-track recording, they say they "overdubbed" it. This is not correct. "Over" means "on top of". There is no overdubbing in a multi-track recording.
I have put my very first overdubbed recording (Chinatown) that I ever made (1955) on my website (Music Download/Archives Tab). It was all uphill from there.
George
George Bowley & Laurie Dupuis
Music in the Les Paul and Mary Ford Tradition