SQL Server Always On Availability Groups help you build a more highly available database server by spanning your database across two or more SQL Server instances. When the primary goes down, the secondary can take over. You can also scale out reads to the secondary servers.
Distributed Availability Groups take this a step further and let you build an Availability Group of Availability Groups, hahaha, spanning a database across different clusters. In theory, this makes version upgrades possible with lower downtime: the individual AGs can be different SQL Server versions, and you can do rolling version upgrades. (That’s the theory: