A Rust object can be owned (T), mutably referenced (&mut T) or immutably referenced (&T). To a given owned object you can create either a single mutable reference or any number of immutable references (concurrently, sequentially is fine).
When you create an iterator from a container, the iterator contains an immutable reference to the container. As long as the iterator is alive, it's not possible to take a mutable reference to the container, and thus not possible to modify it. Because a reference can't outlive its source, this also ensures the iterator can't outlive the collection.
