An Auth0 tenant organizes all the resources you need to provide IAM services. We (where I work) organize tenants around prod vs. non-prod (which I believe is pretty typical), plus temporary tenants for one-off tests, proof-of-concept work, demos etc.
All our (production) applications have clients (in the OAuth 2.0 sense) in a single tenant with a single user data repository. There may be use cases for multiple production tenants, but I know Auth0 ProSvcs folks have steered us away from “multi-tenant applications”.