Mixing and mastering are different stages of audio post-production.
Mixing
Mixing works with individual recorded elements and can involve balancing levels, positioning elements and shaping how the parts work together.
Mastering
Mastering works with the completed mix and prepares final audio for its intended use. It is not a replacement for fixing fundamental recording or arrangement problems.
File Preparation
Clearly named tracks, correct versions and useful notes can make post-production more efficient.