What does a compressor do you ask?
A compressor is a beautiful thing for bass. I think of it as smoothing out all the ruff edges without loosing the PUNCH.
Look at the parameters of a typical compressor:
Threshold - This is the dB level at which point the compression/effect begins
Ratio - the ratio of compressed/effected sound to input signal
Attack - How quickly/slowly the compression/effect 'kicks in'
Release/decay - How quickly/slowly the compression/effect 'lets go'
Output - This is basically the output volume
Threshold:
When an input signal is received- think of this as when you thump your string - there is a definite intensity based upon how hard you hit the string. This intensity decreases gradually if the string is unimpeded (you let it ring). The intensity is measured in dB (decibels). The compressor's Threshold setting determines at what dB level it will start it's job of compressing.
Have you ever thumped the string too hard? A compressor's threshold level will help hide this by effecting the signal above the level you specify.
Ratio:
This is basic. Think of this like the MIX between compressed sound and natural input signal.
Attack:
How quickly does the compression start to take effect? This can be used in different ways... You can eliminate punch by setting this to quickly, but get distorted punch levels if it's too slow. You will have to play with this setting to match your style. Personally, I set this to a very quick level, with a longer release and higher Ratio setting to keep the punch.
Release/Decay:
This is how long you want to hold the compressed signal, for example, if you hit a string too hard and your signal goes above the threshold, the compressor keeps it from "clipping" and keeps the signal within your peak limits. But, as previously mentioned this can loose punch if the sustained signal is also hot. Setting a lower threshold and longer release time will help keep your signal puchy by making more of the signal "even" relative to itself.
I've drawn some quick illustrations to help.
Although this is a very basic explanation, hopefully it will help you discover how a compressor can help you achieve your ideal bass sound.
I recommend the BOSS GT-6B for all of your bass tone and effects needs!!!