0dB means nothing without a reference, which comes after the 'dB' bit. Here's a few definitions for y'all. For those of you that don't know, a 3dB increase is actually doubles the signal level. Try it for yourself, double a mono signal in your DAW of choice and see how much the overall level goes up. It won't sound twice as loud because of the way our ears perceive loudness but the level changes by that much.
0dBFS stands for Decibels Full Scale. That is what your DAW says 0dB is. It means that 0dB will be the highest signal you can have without clipping. Signals half that level will be -3dBFS and one quarter that level will be -6dBFS etc. dBFS only applies to digital signals.
dBSPL is a measure of the acoustic 'sound pressure level'. That's where your 0dB is the threshold of hearing comes from. It is referenced to 20 micro Pascals of pressure. Every 3dB above this is twice as loud etc.
dBA is the same as SPL, but weighted for human hearing (against an A-weighting curve). This means that all frequencies are normalized so that a 40Hz sine tone measured at 60dBA will sound as loud as a 1kHz tone at 60dBa (even though the actual SPL for the 40Hz tone will be much higher).
So most background noise in offices would be something like 40-50dBA. 90dBA is where things are very very loud. 105dBA and up and we're talking hearing damage.
And 1dBA is about the smallest change in level we can hear.
Hope that was useful.