Individual tracks are blended to sit well together. Relative volume, stereo panning, and other aspects are considered, with a focus on the artistic intent.
Gentle adjustments are made to the overall frequency spectrum and dynamic range, and loudness is brought up to a level that is appropriate to the material and listeners' expectations.
Edit / Repair
Audio tracks are cleaned and prepped for mixing (or delivery). Potential areas of concern are clicks, electrical hum, and uneven levels.