Hi uncle_wong_69,
You're spot on, kvass is definitely a fermented drink. The base is traditionally dark rye bread, often stale bread actually. You soak it, then add water, sugar, and yeast, and let it ferment. That's the basic process, BTW.
For flavor, people do add things like raisins, various fruits, or even mint and other herbs. So, no, it's not always *just* rye bread, but that's the core. It's a pretty interesting historical drink! :)