Layering is a fairly simple concept also one of the most powerful that I use all the time to get great sounds. Layering can be used for a whole range of things such as building songs, bringing them down, add impact to specific lines, fill in gaps in the frequency spectrum or make instruments such as bass and guitar sound bigger. Sometimes the sound you need can't be achieved with one instrument, but by layering instruments a whole host of doors are opened.
Many musicians and producers use this technique, so much so people as Steven Slate have created plugins specifically to layer drum samples with drum sounds.
When layering sounds, it's all about finding where the sound is lacking and adding something that fills that need. In the low-end I commonly add sub to baselines. With pads and synths I use a mix of warm and bright pads to give a full range sound, rather than trying to achieve this with one sound. I can add more punch to a kick drum by using an electronic sample underneath. The goal is to see a need and fill it, this way you don't over do it. A good decider is to take the sound away, if you miss it, add it back in, if you don't, it's probably not needed.
Try layering different pianos on different octaves.
Try matching a warm pad with a buzz pad.
Try make a real beefy kick drum.
What happens when you stack acoustic guitars?