What do identifiers with a single quote (') mean in Rust?

I started learning Rust and often see strange identifiers starting with a single quote in code, like `'a`, `'b`, or `'static`. I understand that these are not strings or characters because they appear in angle brackets after a function name or in struct definitions. Here's an example I found in the documentation: ```rust fn longest<'a>(x: &'a str, y: &'a str) -> &'a str { if x.len() > y.len() { x } else { y } } ``` What is `'a`? Why is it needed? And what does `'static`, which I've seen in some examples, mean? It seems to me that Rust uses these notations for something related to memory checking, but I can't understand the logic. Please explain in simple terms, as for a beginner.