To mix music down means to combine several tracks of audio into a single file when doing multi-track recording. The term "render" means the same thing. It's actually more often used these days in many audio software programs.
Multiple tracks can be recorded simultaneously or one at a time. That last option is very handy if you are a solo musician.
You can play a guitar on track one, another guitar on track 2. Then sing on track 3, and sing harmony 1 on track 4. Then you can sing harmony 2 on track 5, and so on until you're done. You can listen to your previous track(s) as you record each subsequent one. It's very cool. But I digress.
Once you have your 5 (or however many) tracks all recorded, and they play back nicely together with each track at just the right volume (because you adjusted the individual volumes of each track - a process called mixing), you're ready to save your masterpiece and share it with the world.
But wait. In what format does the world expect to see my masterpiece audio? Well if you are creating a CD, you probably want a wav file. If you're going to be sharing the audio on the web, you'll probably want an mp3 file. The point is, you're going to end up with just one file. At the moment you actually have 5 (or however many tracks) separate audio files in your DAW project/session.
Your DAW will have an option called either "mix down" or "render" (more common these days). What this option does is combines the output of all your tracks, at their current volume relative to the other tracks, into one audio output.
It's like putting lots of things (like 5 audio files) into a funnel, and having only thing (wav or mp3 file) that is a mixture of the 5 sources come out the small end. See the picture on the right, which pretty much sums up (no pun intended...OK, pun intended) mixing down.
The term to mix down comes from the idea that you have many bits of audio spread across many different tracks when recording. But ultimately you'll need to combine all that stuff into one audio file that someone can listen to.
Even in the old days, when we were recording on very wide strips of magnetic tape, a mix-down session would happen at some point.
Then the recording engineer would transfer all that audio onto a 2-track tape (one "track" for the left channel and one for the right channel for stereo). That tape was much much thinner and could be played back on some consumer machines (you did own a reel-to-reel machine back in the day didn't you?).
But more importantly, these tapes were used on the machines at mastering houses. See our post - Mastering a Song – What Does It Mean? for more on what "mastering" means. The mastering engineers would then transfer that mixed down audio to records or cassette tapes.
The phrase "mix down" is actually kind of phasing out since most people do so much stuff on computer now. I've mentioned a few times that people now often use the term "render" to mean the same thing as "mix down."
I believe this is at least partially because we use the term rendering when doing the same basic process in video production. And since there is now a lot of cross-over between video and audio in the software of both worlds, terminology is becoming more standard.
Just know that if you see the term "render," it means the same thing described in this article as "mixing down."
You can learn how to mix down your own audio, as well as tons of other awesome audio tips and tricks, in Reaper software in the Newbies Guide To Audio Recording Awesomeness 2: Pro Recording With Reaper tutorial course. Now go forth and mix down or render your masterpiece.