Samsung is in the process of redesigning the software on its Android devices. Little by little, the company's phones are getting updated from Pie, and OneUI comes with it. It also offers new official apps, like the latest update from Galaxy Apps. Oops, it's "Galaxy Store" now.
The newly-renamed Galaxy Store is Samsung's showcase for apps and games, the most important being Fortnite. This is also where you will get apps for Tizen based wearables. The functionality seems unchanged, but the Galaxy Store has all the usual OneUI curves. However, users report that it does not work with Samsung's dark mode in Pie. This seems to be a major oversight.
The updated store is being deployed and is a required update. The older version will not launch anymore. You can also update in sideloading the new version from APK Mirror. This is v4.5.01.7. In addition to the name change, the application icon is a little different. Do not hesitate to compare old and new on APK Mirror.