"Compression" is an audio treatment or "effect" that is often hard to explain to someone new to audio recording. Actually, the basic idea of compression isn't very hard at all; it's just too often poorly explained, like a LOT of audio recording concepts. I've given it a shot a couple of times here on Home Brew Audio - see Improve Or Ruin Your Audio With an Effect Called Compression, Should You Use Compression In Audio Recording? and Vocal Compression Using Reaper’s ReaComp Effect Plugin. Those last two have videos to help explain compression.
One important thing that I think gets left out of a lot of explanations is that compression turns the audio volume down, but only PARTS of an audio file. Only the audio that is loud enough that the volume crosses a line (determined by you) will get turned down; all the rest of the audio remains as it is.
Then, once only the loudest sections of the audio have been turned down, you can (but only if you want to!) turn ALL of the audio up a bit louder than you could have before compression. I think this concept is much better shown than explained in words, and I do just that in the video at the What Is Audio Compression and Why Should I Care? article, which I mentioned above.
Anyway, the reason I started this post was to tell you about a series of posts and videos explaining compression that is being done by the folks at Sonic Sense. Here is a video that accompanies it is below. Enjoy!