"B389" apparently refers to Apple's rumored tracking tag hardware that will work much like the Bluetooth tags from Tile. By tapping the "Add B389" bar, you'll be able to tag and track your bicycle, briefcase, keys or any other objects you want to keep safe. The physical tags could be simple white disks with an Apple logo in the center, according to a previous MacRumors article.
iOS 13 users will be notified when they part ways with a tagged item, according to code in the iOS 13 build. To find it, they can tap a button that will make the Apple tag beep loudly. If that doesn't work, the item can be placed into "Lost Mode." If another iPhone user finds it, they'll be able to see the owner's contact information and phone or text them. Users can set up "Safe Locations" where an item can be left without the need to notify the owner.
Previous reports show that the Apple's tracking tags might be powered by a new co-processor in the iPhone 11, and use a combination of ultra-wideband and Bluetooth LE to narrow down search areas. It might also let you find devices using AR, and show red and orange balloons to indicate when you're getting close. "Walk around several feet and move your iPhone up and down until a balloon comes into view," a prompt from an internal iOS 13 build reads.
MacRumors notes that the build containing all these features is from June, so the final version of the app might be quite different. Apple might not even release the app or tags, though at this point, it seems more likely than not to become a product. If so, we're likely to find out when Apple officially launches iOS 13.