Please see the documentation.
This crate provides rust collections that are 'functional' (or persistent or applicative or shared or immutable) under the hood. Clone operations are cheap because a clone of a collection shares its represenation with the original collection, at first. As the clone or the original are updated, their internal representations will diverge, although I attempt to retain sharing and minimize new allocations.
The "immutablibity" is in the internal data structures. I attempted to match the interface to the API of the standard rust collections, including by providing mutating operations. When one of our collections updates, it clones its internal state as necessary, namely when that state is shared with other collections. Thus, the state of the collection prior to an update may persist (in another collection), but the updated collection may use retain only part or none of that original state.
While persistent data structures are the norm in functional languages, their use in a language like rust is likely to be niche. For most uses, they will be much slower than the standard libraries. Their best use case is when you need to keep many clones of the collections. For example, this may be the case when building a symbolic execution engine that explores many different execution paths in a breadth-first manner. Each explored path needs a symbolic memory. By building the symbolic memory using persistent maps, the symbolic memories for paths with a common prefix may share much of their representation. This can result in dramatic savings it space and time.