A regional disk is a logical disk that is synchronously replicated at the block level across exactly two zones within the same region [1]. Since the disks are always identical, with no replication lag, the HA control plane can seamless fail over the whole database to a new master that plugs into the same disk. It's all in the article.
Regional disks aren't publicly available yet, but they are in alpha [2]. Like normal persistent disks, everything is backed by Google's internal Colossus system [3].