This question is difficult to answer because it's likely that the most compex concept you think you understand you don't really. I thought about this question

This question is difficult to answer because it's likely that the most compex concept you think you understand you don't really. I thought about this question and thought, Rust lifetimes or Rust coherence rules, but then again I thought maybe I don't really understand them. Perhaps it is more helpful to ask: what is the most complex concept you can explain the basics?