It isn't totally obvious that you need compression per se, but it's as good a method of normalising the volume as any...

Compression is usually used to reduce the dynamic range of a track, ie the variation between loud and soft passages. It does this without changing the overall 'shape' of the waveform though.

The first stage of compression is the bit that makes loud passages quieter.

The threshold level defines the signal level at which the compression will kick in, in this case shown at -20dB; now in a hardware compressor, that dB level would usually be with respect to line level, in SW, I'm guessing that 0dB refers to the maximum signal magnitude that can be represented.
The peak/RMS toggle defines the method used for calculating the current signal magnitude. For reducing clicks, thumps or anything else that is transient, peak might be a good choice. I'd probably try RMS first for your needs though. (Although since we've already suggested that you don't need to compress the music, it shouldn't make much difference.)

Ratio defines the amount by which the magnitude of the signal will be reduced once it goes over the threshold. Compression is dynamic - the more the signal goes above the threshold, the more it will be compressed.

Attack defines how quickly the compressor will 'kick in' when the magnitude raises above the threshold. Similarly, release defines how long the effects of the compression will hang around after the magnitude drops below the threshold.

The post gain is what makes this useful to you - we know that the threshold and ratio settings have given us headroom, and the post gain will expand the signal into it, without clipping.

But enough of the technicalities - leave attack, release and ratio at those settings as they look reasonable. Work on the threshold/post gain first...Actually the settings there look reasonable as a starting point too.