In a multi-AZ configuration, AWS will provision a standby MySQL instance in a separate Availability Zone. Changes to your database will be replicated to the standby DB instance in a synchronous fashion. If there is a problem with your primary DB instance AWS will automatically failover to the standby, promote it to be the primary DB, and provision a new standby.
You don't have access to query standby databases directly. So you can't use it to handle all of your read queries, for example. If you wish to use additional database instances to increase read capacity, you'll need to provision a read-replica. We'll cover those in a separate recipe.
Backups will always be taken from the standby instance, which means there is no interruption ...
