match number_str.parse::<i32>() {

This matches on the return value of parse, which is an Result<T>. It can have two values: Err(E), or Ok(T).

        Ok(n) => Ok(2 * n),

This line matches an Ok(T), and binds the value of type T to n, which can then be used on the right hand side.

        Err(err) => Err(err),

this line matches an Err(T), and binds the value of type T to err, which can be used on the right hand side.

The right hand sides re-wrap these values into another Result.