We have a few articles about the use of an effect called compression in audio recording. See our first one here: Should You Use Compression In Audio Recording? Well usually the compression settings (like threshold and ratio) are applied to the entire frequency spectrum of an audio item. But sometimes it might be useful to apply different settings to different frequencies. For example, you might want to really squash the low frequencies (say, a "band" of frequencies between 60 and 200 Hz), but have a faster release on them, while at the same time, applying subtle compression to the middle group (band) of frequencies. That is called multi-band compression.
Our article on multi-band (or more often, multiband) compression explains in more detail. But we like to offer different points of view and perspectives.
Here is an article from Audio Geek Zine that shows you how to use multi-band compression, along with audio examples of drums and guitar recordings. check it out here: http://audiogeekzine.com/2012/09/how-to-use-multiband-compression/