Mixdown
There are a lot of techniques that can be used during mixdown that can have a big effect on the sound of the finished product even though all of the recording is done. Here are some techniques that you will want to think about as you work with your recording engineer (or on your own studio.)
Faders
At the end of a multitrack recording process, each track will be recorded as best as possible as a raw part. It was recorded at the best volume level for that instrument at the time. At the end, however, it needs to have the correct volume and panning in relation to all of the other parts.
For example, the vocals should be clear and understandable which doesn't happen if the guitar is twice as loud. Since a voice is often less loud than a guitar, the vocal is often turned up during a mixdown and the guitar is turned down.
Sometimes, the volume level on some instruments needs to be ridden up and down during mixdown. This could be because the instrument wasn't consistent during recording (or parts were recorded on different days and had different volumes.) Or it may be that you want to emphasize a particular instrument on instrumental parts and turn them down a bit during the verses while the vocalists are singing.
Setting the faders properly for each track is the main job of the mixdown process.
Panning
Panning steers the output of the track between the right and left speakers in a stereo recording. In the very early days of stereo, the unsophisticated engineers used to put some instruments entirely on the left or right speakers. They caught on quickly, however, that normally, some instruments and parts should be slightly left or slightly right to give it a more full sound.
For example, if you have two background vocals, often they will be panned off to opposite speakers to give the idea of two people standing on either side of the main vocal, which is usually centered.
The panning is set during mixdown to be mixed onto the final right and left stereo tracks.
EQ
EQ stands for equalization and is one of the harder concepts to describe yet is one of the easiest to demonstrate to people. Good use of it can mean the difference between a good and a bad recording.
As you record an instrument, the microphone, and the recording equipment itself catches different aspects of the sound. Each sound has a low, mid, and high set of frequencies. An EQ unit can boost or suppress different parts of these low, mid, and high parts to the sound. The result, if done right, is a "clearer" sound.
Each track should be EQed according to the best sound of the instrument. Usually this is done by soloing that particular part so that there are no other distracting sounds in the recording, then fine-tuning the EQ to sound best. Most instruments sound a bit better when some of the mid range of frequencies are taken out and highs and lows are emphasized. Generally, most good EQ's look like a smile if you see it on a graph. This is a good place to start for many sounds with a little bit of tweaking from there.
One technique that is used with EQ is to make the best EQ for each instrument, but, when done, make a "hole" in the range that the main vocal is recorded (assuming that there is a vocal track.) This lets the main vocal stick out without having to just turn it up louder. This takes the “clutter” out of that range of EQ. When doing this, each particular track might sound pretty bad alone, but within a mix the main vocal will sound clear and you'll be able to hear the background instruments more clearly too. Think of it like an EQ jigsaw puzzle. You will want to make room for the vocal piece.
Post effects
Most effects on tracks are done during mixdown rather than on the original recording. A vocal track, for example, might be recorded "clean" but then might have distortion put on it during the mixdown process.
There are many other effects that are often put on during mixdown including gates, limiters, phase shifters, pitch shifters, and a whole host of other effects. There are books about the best use of these depending on the situation and they are best learned while playing with an effects processor on a particular track.
There is often special attention played to the drum tracks during this process. For example, gating the snare can give it an extra "kick." And some mixing software and boards have the ability to time shift particular tracks by milliseconds which can actually adjust for the speed of sound to the microphones. This can make the drum impacts more powerful on your mixdown.
Clearly, this small section doesn't do this topic justice. It should give you at least an idea of what happens during mixdown so that you are aware of what is happening. If you are not a recording engineer yourself, it's best to use someone experienced to help you with this. But knowing about it will let you start to talk about it with them.