During mixing, all of the recorded elements are balanced, processed, and arranged in the sound-stage (the space from left to right when using stereo speakers) to create a clear and finished version of your performance.
The editor sends your tracks to a mixing engineer (usually a separate engineer who specializes in mixing). The mixing engineer uses compression, reverb, delays, and other processing to fit everything together so that everything is heard at the right amount. They create an atmosphere for the track that fits the style of the song or your performance. The mixing engineer will also enhance the most important elements of the mix (the drums, bass, and lead vocal) so that they are easily heard.
Mixing takes many years of practice to gain proficiency and requires that the engineer have significant knowledge of the physics of sound. A mixing engineer that works with bands has to make sure that many competing voices and instruments in a mix have their own space and can be heard. This can be hard to do when many instruments sit in the same frequency range. Acappella mixing can be even more difficult because you're dealing with many tracks of the same instrument (the voice) stacked on top of each other, competing for the listeners attention.
I've heard several Grammy-winning mixing engineers say that they are still learning and struggling with every mix they do and that they change parts of their approach regularly as they continue to learn and improve.
Like editing, this step is better left to those with experience. It will save you time and money.