Depends on how you use it..
I am a pianist and I play for a jazz band and when we use the term modulate, it means to change keys, to either go up or down a step
But in this case modulation means how much something is getting affected by a certain effect, 100% or 15%, it varies. For an example if I have a kick bass quarter note sequence and I want the first beat to be the loudest, you would set your modulating volume high, and turn the modulation of your FIRSt kick to 100% , so that it will be completely affected, and your other beats to lets say 0% so it wont be affected at all.
Hope that helps! Im TopHat btw :D