Mixing is working with the individual tracks, separating the vocal from the instrumental, each instrument from the other. A mixing engineer will normalize the volume so no track overshadows another, all can be heard. But more importantly the engineer equalizes the tones of each track to bring each to its highest form. You want the vocals to shine, the drums to pop, that awesome lead guitar lick to carry, and each to pan the stereo spectrum for a full, wide sound. This is the mixing engineer’s job.
Mastering is the art and science of bringing the mixed tracks together and optimizing the sound on releasable media. Whether your release is to be CD, high resolution for streaming, or old-fashioned vinyl, the mastering engineer gives you that final professional sound.