Basically, decibels are measurements that express the magnitude of a physical quantity (ie, power or intensity) relative to a set reference level.

That said, 0 dB is typically used as the reference level when dealing with acoustics, mostly because 0 dB is what a typical human considers "normal."

To put things into perspective a bit, normal conversation between two people is anywhere between 40 – 60 dB.

So by having your signals/tracks closer to 0 dB, you'll avoid clipping (the signal is too "hot" and makes things distorted and fuzzy sound). Since compressors change the sonic dynamics of the signal, if your track is currently pumping out at 20 dB, if you compress it, depending on the settings, you could cause clipping/distortion, or it could make your track seem quieter (again, depending on settings of the compressor). Also, keep in mind that compressors could possibly cause a lot of noise/hiss on your track while being used, and you'll often see a compressor paired with a noise gate/filter.

To do this in GarageBand, I have no idea.. I don't use GB, so some one else might have to help you out. I found THIS link might help you out a bit more with the actual functionality of GB.

Good luck!