A short post for devs who are not musicians.
Mixing is making a song or sound effect sound good at any volume on any playback device. There's a bunch of technique involved, but what matters is this: just because a song sounds OK to you on your headphones doesn't mean it will sound good on someone else's cell phone or home entertainment system. What do I mean by "sound good?" Simply put, the audio image is blurry and not as impactful as it could be.
Imagine if the colors on your computer screen changed, drastically, depending on the shape of your room and the dirty clothes on your floor. That's acoustics. The playback device and the room are a system, meaning they cannot be meaningfully isolated from each other.
Also, the balance between sounds in the frequency spectrum change unevenly as volume increases. Broadly speaking, sounds towards the top and bottom change in volume faster than sounds in the middle. At loud volumes they are louder than the mids. At soft volumes they are softer. If a particular component of the audio image - an instrument or part of a sound effect - has more extremes than mids, it moves into the foreground of the audio image as volume increases. This distorts the composer/sound designer's intention.
The mixer works in the closest he can get to an acoustically neutral space and tests the mix on a wide variety of devices at all volumes to ensure the accurate transmission of the composer/sound designer's conception. Done well, the mix is effective in the widest circumstances.
And now you know what your audio guy is doing when he's mixing.