Anakin Starkiller said:
Super late, but…
Over-dubs are additional recordings that play over other recordings.
For example, most guitarists will record their guitar track, then a second identical one that plays over the original to create a bigger sound. Vocals, drums, ect. are done the same way. It’s called over-dubbing and the additional tracks are called over-dubs.
That Queen song has a ton of different vocal tracks going on is what it’s saying.