Decibels, more commonly abbreviated as "dB," are a measure of audio loudness and energy and something we talk about a LOT in audio recording as well as live audio. You can't say a lot about equalization (EQ) or volume, especially relative volume (how much louder or less loud something is that it was before or than other tracks, etc.) without referring to dBs.
The problem (especially if you were not a math fan in school) is that doing any kind of calculations involving decibels is most definitely not straightforward. dBs involve logarithms (logs). Yeah, that's right. You remember the scary button on your calculator with that word on it? Well if you're going to do any dB math, you gotta deal with logs.
Fortunately, Chuck McGregor just wrote this excellent guide to doing dB calculations without the pain. See the full article here: http://www.prosoundweb.com/article/a_practical_guide_to_key_audio_calculations/?utm_source=feedburner&utm_medium=feed#When:18:34:18Z