The specialization did a good job of explaining all of the basic concepts of RL in a fairly understandable way. The instructors don't assume a huge amount of prior knowledge, and make it pretty accessible. They split reasonably between theory and applications. I would say that you need some background in math and ML.

The courses do build upon each other. You could do just the first course and get a good overview of RL without diving into the details, but I don't think you could start midway through the specialization.

I would also suggest looking at the Hugging Face Deep Reinforcement Learning course. That one is very different (focused more on application than diving deep into the theory), but it's taught by a non-academic and really tries to explain the concepts in a way that is approachable to most programmers.