A mix that leaves the room.

Vocals, instruments, and space balanced so the song plays in the car and on a phone, not only on the studio monitors.

What a mix covers

A pass on the session: what is fighting, what is missing, and what should sit forward.
Vocal level and tone that stay intelligible without flattening the track.
Drums, bass, and the low end checked so the song holds together at volume.
A revision round against your notes, played back in the room.
A stereo mix and stems when the next step is mastering, here or with someone else.