A mix bus is where all combined tracks are routed and merged together for editing. If you just want to organize your tracks in sub-groups or set up complex effects audio flows with auxiliary buses, mix buses can be a huge timesaver.
The most popular mix bus is the basic main stereo mix bus. It serves as the master control center, where you can make final adjustments before the audio leaves your board and hits the speakers. Moreover, there are two other types of mix buses that can significantly help in your mixing process: subgroups and aux tracks.
Read more about this here: https://www.izotope.com/en/learn/mix-buses-101.html