Seeking help / tutorials for Animator Controller

Hello all,

I'm sure there are tutorials out there that Uncle Google / Youtube just weren't able to help me find, and I'm soliciting the community for links to good ones.

I'm at the point where I'm trying to learn the nuances of the Animator Controller. I've watched some videos, understand how to add animations, transitions, booleans, etc., but not quite wrapping my head around WHY things work. To wit:

I know that if my main animation is idle, that will play. And I know that I can make a transition from idle to say "walk" (well, really locomotion, since it contains both walk and run) where the prerequisite is that my "speed" bool is greater than .01. My question is, how is that working, or how is it set off? Is it something like my AI script calls for the npc to follow me at speed 4 when I get a certain distance away, and that in and of itself triggers the "walk" animation, or is it more than that? That's the most "makes sense" thing I can think of, but then I'm wondering how to initiate battle animations and the like. Sorry for the noob (stupid) question. Like I said, I figure there are tuts out there that cover the why of things, and I just haven't found them yet. God bless.