It's not easy without a bit of maths.
Basically, the term dB doesn't mean an absolute; rather, it's expressing the difference from a certain known value and the letter following the dB tells you the starting point.
Some of the common dB terms in audio are:
dBV represents the level compared to 1 Volt RMS. 0dBV = 1V. There is no reference to impedance.
dBu represents the level compared to 0.775 Volts RMS with an unloaded, open circuit, source (u = unloaded).
dBm represents the power level compared to 1 mWatt. This is a level compared to 0.775 Volts RMS across a 600 Ohm load impedance. Note that this is a measurement of power, not a measurement of voltage.
dbFS - relative to digital full-scale.
dB SPL - A measure of sound pressure level.
The two you're going to encounter most often in home recording are dBu and dBFS which are the scales usually seen on meters.
The meters you see on mixers etc. are usually in dBu with the .775 volts RMS I mention above usually being equivalent to the 0dB position on your meters. (I say usually because this isn't always true--depending where you are and what standards you follow, it might be +4dBu or something else...but that's for lesson 2). Anyway, most decent gear is built so the signal can go considerably higher than 0dBu/.775 volts RMS. Typically, things can handle at least +18dBu before clipping, sometimes more.
This brings me to dBFS (which stands for full scale). 0dBFS is the point in a digital system where you run out of bits and digital clipping (which is nasty) starts.
Since the headroom in analogue usually gives you to +18dBu, the norm is to assume that -18dBFS is equivalent to 0dBu on meters on the analogue side.
Remember this conversion and you have 90% of what you need.
That's what I do. I drink and I know things.
-Tyrion Lannister (and Bobbsy)