Think of a drum map as a translator. It takes the input note and redircts it to an output note which can be the same as the input note or a different one as you see fit.
Each line in the drum map can also be assigned to a different MIDI output port, and even channel if you wish.
Although intended for making multiple drum synths available from one track they can also be used to map non-GM drum synths to GM keys and allow greater control within Sonar - such as being able to mute or solo individual drum notes within the PRV.
If you have the SWA Complete X2 video there's a chapter in there on writing drum maps with a full explanation on how they work.
Take a look at one and concentrate on one line - you'll see that they are relatively straight forward.
In Note----->Out Note----->Channel------>Synth