Normally when you add a database to an Availability Group (AG) you need to start with the database on the primary node. You restore backups
With Norecovery on the secondary (either manually or with the GUI), add the database to the AG on the primary, alter the database on the secondary to reference the AG, and your job is done.
Sometimes after all the preparation you discover the wrong node is the AG primary, and you can't justify failing everything over just to add one new database in. But if you can tolerate a short outage to the database you're adding, then you can add it to the AG "in reverse" using a tail-log backup!