What is diabetes and how does it work?

A co-worker and I were talking today at work. There's a different co-worker who has type 2 diabetes, but told me they changed their diet and exercised, listened to their doctors advice, and became pre-diabetic. Anyway, we were then talking about, I said about if your body doesn't make insulin, then you need insulin shots, but my co-worker is thinking that the body has an incredible way of healing itself, or rehabilitating, but my thought was that, if your body no longer produces insulin, like you can't ever reverse that.

Then we were talking about, what exactly is diabetes? Like, do you have to watch what you can eat and drink, what your sugar intake is? And then my co-worker said, but, don't some diabetics get like in trouble if their blood sugar gets too low, then they need sugar? So I was left a bit confused.

I know I can just look all this up on google, but wanted an answer from someone who maybe has it or might know something more about it than I do.

Will update this post as needed.