In our recent article about compression in audio recording, you learned what compression actually means. There's even a video (but don't pay attention to the messy book case, which has been cleaned up since then)! Basically it is a way to even out the loudness levels in your audio. This is sort of like using a trash compactor on your audio waveform, which then allows you to turn it up louder before the loudest bit of audio hits the limit for maximum volume.
But why would you want to do this? Is having louder audio always a good thing? Well, that depends on who you ask. And you definitely don't want to over-use compression because too much tends to give you odd audio weirdness like pumping or extra sibilance. But here is an article that will show you some ways to use a compressor for extra punch and presence in your audio. The article focuses a bit more on music than voice over recording, but the concepts are the same.
Read the article here: http://www.prosoundweb.com/article/guerrilla_recording_compression/