... but what is an "active" snapshot?
Sorry if I'm confusing you. Don't worry too much about "active". The way snapshots work in VMware products, all snapshots in a chain (i.e. one or more snapshots) are basically actively used in Read-Only mode.
If one of these snapshots had a reference to a datastore at the time it was created, VMware won't allow you to remove that datastore, because of the option that you could later revert to this snasphot, which may require access to files on that datastore.
André