depends on the context
general catch-call explanation would be: reassignment of midi messages to a more convenient layout
in the context of the Axe-Fx, there are two things that could be considered "mapping":
- the simpler one is the "MIDI PC OFFSET"...
This takes whatever PC message you send to the Axe-Fx and re-interprets it with an offset of whatever value you choose. So, an offset of 7 would alter PC#15 messages to call up patch #22 instead (15 + 7). A convenient use for this would be to have, say "studio" presets in Bank A and "live" presets in Bank B. Then, when going live, set the PC offset to 128. You then don't have to reprogram your MIDI controller to call up the appropriate patch. Unfortunately, you can't set it up for a PC offset of 256 (a limitation of the size of the actual memory bit used) so you can't quite do the same thing with Bank C.
- the more complex one is the Axe's actual "MIDI Mapping" feature...
There are three parameters for this one, though it's slightly more complicated than that. MAPPING MODE toggles the entire mapping feature on or off. MAP FROM allows you to select each of the first 128 PCs (0-127) and then re-map it using MAP TO to a different Axe-Fx patch in the range of 0-254. Using this, PC#0 could call patch 27 while PC#27 calls patch 54 while PC#23 calls patch #239, etc.
Now... why would you want to do this? Good question. One usage I can think of though, is if you want to use the same patch for multiple songs in your set. You don't have to waste slots on the Axe-Fx copying the same patch into multiple slots if you want to set unique PCs for each song. Or, if your controller isn't particularly flexible and can only go in order 0-127, or something like that, you can also re-map things there. It's also a decent way to access Bank B without having to go worry about jumping through the CC#0 MIDI Bank Select hoop.
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Lastly, as for why it "depends on the context"...
I have no idea if you were referring to mapping on the Axe-Fx or on the GCP. I don't know much about the GCP, and although I've explained MIDI mapping as it pertains to the Axe-Fx, it's still entirely possible you were intending to have some opinions about MIDI mapping on the GCP and everything I just typed, while still applicable to "mapping" won't help you.
Likewise, if I were talking about MIDI mapping with my drum software, I'd be talking about how MIDI Note On/Off messages correspond to different kit piece articulations in the virtual drum kit and how I might re-map them depending on if I'm controlling the drumkit from a keyboard, an e-drum kit, or a controller like a Zendrum.
In any case... hope it helps!
