Storage classes describe different kinds or classes of storage to use with different services, service levels or backup policies….

I think they describe primary persistent storage, but I am not completely sure about that. Perhaps there are classes for ephemeral storage, too.

Storage classes describe different kinds or classes of storage to use with different services, service levels or backup policies

More important, it enables dynamic storage provisioning.

In addition to what @JMeixner, a Kubernetes StorageClass enables you to dynamically provision persistent volumes (PV) in a Kubernetes cluster. Kubernetes administrators define classes of storage, and then pods can dynamically request the specific type of storage they need