So, the regular Galaxy S5 is finally starting to get an update to Android 5.0, and what's this? The GS5 Active on AT&T is getting an update. Could it be? Is this the moment we've been waiting for? Oh... nope. It's just Android 4.4.4.
Here's the changelog. As you can see, it's highly descriptive.
Android KitKat 4.4.4
Miscellaneous improvements, bug fixes, and security updates
You can head into the update menu to manually pull the OTA, otherwise it will show up in a few days. The update is mandatory, and will eventually force installation after you've postponed a few times. Don't want Samsung's crummy KitKat update? Tough.