"The technologies that define today's Mac experience-such as Metal graphics acceleration-work only with 64-bit apps". The support page broadly explains Apple's plans to "eventually" require all Mac software to be 64-bit. Further, it also includes a previous warning from Apple to the developers that High Sierra will be the last version to use 32-bit apps.
Only a day or two ago, similar notifications were seen by watchOS users, and it seems Apple is on a roll calling developers to improve compatibility of apps. On the other hand, pressing OK will close the alert box and launch the app normally because the current macOS version still has support for 32-bit apps. Aside from the absence of support for 32-bit apps, the new Mac operating system is said to come with performance improvements, bug fixes and new features. If you don't update, the app will not run in a future version of macOS.
Apple is making the transition to 64-bit in part because it will be easier to maintain, and also because 64-bit apps are more efficient in most cases. This gives them time to prepare accordingly, though 32-bit apps will continue to run and function properly for the time being. When you select an individual application, you will see a field titled "64-bit (Intel)".
Apple Begins Notifying macOS Users That 32-bit App Support Is Ending
In the left column of the window that appears, scroll down until you see the Software section. "Yes" indicates 64-bit; "No" indicates 32-bit. It will also support universal apps or apps that will work across Mac and iOS devices, according to MacRumors.
How do I get in touch with the app developer?The A7 chip, which powered the 2013 iPhone 5S, was the first 64-bit mobile processor. Make sure future releases of your app are 64-bit compatible by using new diagnostic tools in Xcode 9.3 beta and testing on macOS 10.13.4 beta. The pop-up says that the app needs to be updated. Though the company hasn't been equally aggressive with the Mac users yet, given the warnings have already started it, won't be long before 32-bit takes its last breath on the Mac's as well.
This break with compatibility for the very first version of watchOS is an indication that Apple intends to keep rushing forward with Apple Watch, and that watchOS 5 will be more than just a minor upgrade. Second, desktop apps are downloaded from several other sources apart from the MacOS App Store.