A mixing engineer takes the raw recorded tracks — vocals, drums, bass, synths, guitars — and combines them into a polished stereo (or surround) mix. This involves adjusting levels, panning, EQ, compression, reverb, delay, and other processing.
The goal is clarity: every element should be audible, balanced, and serving the song. A great mix makes a track feel wide, punchy, and emotionally compelling on any playback system — from phone speakers to club PA systems.
Mixing is distinct from mastering. Mixing works on individual tracks within a session; mastering works on the final stereo bounce.