One thing to add to what others have said, when you're met with code like this in the wild and you need to modify/add/remove something but don't have a 100% understanding yet, the common workflow is that you explore this code with your "evaluator" (basically a REPL connected to your editor).
So if you come across snippets of code you don't understand very well, you place your cursor at the various forms and execute them. So you'll start at the inner forms, and slowly work your way outwards, and after that you've verified assumptions both about the function's internal workings and how you'll use it from the outside.