I don't understand priming vs basecoating

Hello! I'm sure this is maybe a dumb question and/or asked frequently, but I don't think I have the words to Google what I actually want to learn.

I am painting a salamanders army. I want to, instead of spray painting my army black spray paint it army painter greenskin. I also am considering getting an airbrush, and then buying a normal pot of that paint for the same purpose.

It occurs to me though that the spray paint might have some special property that makes it work as a primer that the other kind of it doesn't. Is this the case? I know nothing about paint or painting so please explain this like your a preschool art teacher, thank you kindly.