No explanation of type str in chapters 1 - 4.2

[fktyf-13]

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URL to the section(s) of the book with this problem:https://rust-book.cs.brown.edu/ch04-02-references-and-borrowing.html#permissions-are-returned-at-the-end-of-a-references-lifetime

Description of the problem:

I read without skips. I came to ch 4.2, section Permissions Are Returned At The End of a Reference’s Lifetime
and in quiz found function definition:
fn get_first(v: &Vec) -> &str {

What is str? I looked throw previous chapters and found no explanation.

Suggested fix:
Suggest to give explanation.

In general, the chapter 4 is full of complex concepts which are explained with examples with unknown types. It is very confusing. Don't use so many unknown types, or (at least) give their index with short explanations before this chapter.