It's explained further below that section. It will look for those two relative paths in a directory that is the the path of the source file that contains the `mod ...;`, concatenated with a path build by concatenating all nested `mod ... { ... }` statrments the `mod ...;` is contained in.
~~~
// src/foo/bar.rs
mod a {
    mod b {
        mod c; // will look for src/foo/a/b/c.rs and src/foo/a/b/c/mod..rs
    }
}
~~~