Welcome to one of the most flexible parts of the Rust standard library. Iterators are, as the name suggests, a way of applying actions of items in a collection. If you come from C#, you will already be familiar with iterators because of Linq. Rust's iterators are kind of similar, but come with a more functional approach to things.
Because they are an extremely fundamental part of the standard library, we are going to dedicate this recipe entirely to a showcase of all the different things you can do with them in isolation. For real-world use cases, you can simply continue reading the book, as a big portion of the other recipes features iterators in some way or another.