Your sound mixer is an expert in principles of audio / music / sound. In the context of video projects, they usually take the rough audio edit the video editor has created and make a variety of adjustments / changes to make sure audio is leveled correctly and that certain elements of the soundscape are highlighted. While sound editors / designers are responsible for telling a story with music and sounds, sound mixers are responsible for making the sound edit mesh together smoothly.
Since they're usually one of the last parts of the post-production process, good sound mixers are efficient, but meticulous. They are able to taking direction, but make judgment calls on the fly to enhance the audio files you've provided.