There is a new edition of the book and this is an old link.
Every reference in Rust has a lifetime, which is the scope for which that reference is valid. Most of the time lifetimes are implicit and inferred.
{ let x = 5; // -----+-- 'b // | let r = &x; // --+--+-- 'a // | | println!("r: {}", r); // | | // --+ | } // -----+Run
Here are the relevant sections in the new and old books: