Yeah, you've got the gist of it. The simplist bit of MIDI is a message. That's a binary on-off signal encoded in 7 bits (the 8th being a check or status bit), which translates to 128 distinct messages (2^7 = 128). Generally, 0 is all the way off and 128 is all the way on with a linear progression.
Each message gets transmitted on a channel. Every MIDI link has 16 channels. A channel broadcasts whatever message to whatever is listening on that same channel. That might be your DAW, or a keyboard, or your DAW and
a keyboard. You can also have system-wide messages (Omni mode) that broadcasts out on all channels. Setting your MIDI channels correctly is a big part of getting everything talking like you want it.
In the old days you routed MIDI messages by using channels. If you press the C key on your keyboard, that'd send out a C-note message on whatever channel, to every single outgoing MIDI port. That means that C note might head out to both your outboard sampler and your drum machine. If you didn't want the drum machine to get the C, you'd change it to listen on a different channel, and you'd have to program something else on that different channel to trigger it.
Now, through the magic of computerboxes, you can use something like your MOTU to skip the whole channel conflict and route the MIDI you want to specific outputs. So you can say "all this crap going out over channel 1 should only go to port 3, which goes out to my keyboard". And then you could route some other set of MIDI over channel 1, but only have it go to port 6 and 7, which are your drum machines. It also means that you could have everything on channel 1 going out to ports 1-8, but channel 2 only goes to port 3 and channel 10 goes to ports 6 and 7... That means you can have an absolutely insane amount of MIDI flying around, because you can have all 16 channels pumping around separately to 8 different outputs for a total of 128 channels at once. You'd have to do some really contrived orchestration for that to ever be an issue, but it does allow a lot of flexibility without having to swap channels around all the time.
So yeah. Did I just confuse the matter?