Put simply, layering is the act of using two pieces of audio with the same pattern, and placing one on top of the other.
Now a band tends to create this effect just by playing. Consider a band with a horn section. 3 or 4 guys, all playing the same tune, but since they are using different instruments, and are never going to be perfectly in sync, they produce a much richer sound than someone playing alone. A choir or barbershop quartet does the same thing.
Layering Guitars Parts
In rock music they often use multiple takes of the same guitar riff and layer them on top of each other to produce a richer sound. If you are a computer based musician/producer, how precisely do you go about doing that? Well… it depends on how you make your music.
A nice trick for heavy metal guitars is to use two copies of the track and apply distortion on one. Depending on how “dirty” you want it to sound, you can make the clean guitar or the distorted version louder in the mix.
If you play a synthesizer, all you have to do is play the same tune with different instrument settings. If you use loops, you can make duplicates of them and give them slightly different effect settings, alter the pitch slightly, and move them just a bit out of sync.
Loops are easy to layer, but are far less dynamic than being able to use a synthesizer. The reason being that it’s nearly impossible to find two loops of different instruments played the same way, unless you make them yourself.
Another trick is to play a loop and record it, then layer the recording on top of the original.
Vocal Layering Tips
For vocals, depending on the style, several different versions of the same take can be used together to create a self-harmony. Some heavy metal vocalists that use different styles of vocals often layer them together throughout a song.
For example, higher pitch singing/screeching often has growling or low singing layered under it to give it depth.
Layering is one of the most important aspects of creating songs with rich tones. Using the technique the right way can add depth to your songs, but using it too much can make them sound crowded.
Figuring out how far to take things is something you will need to find out for every song you produce.