iOS 11 won’t run any 32-bit apps. Most of the time, that won’t make any difference — most apps you use every day were updated to be 64-bit a long time ago. But we all have a few of those old apps laying around that haven’t been updated in years. Perhaps they’re still useful for you, or maybe Apple kicked the app out of the App Store and there’s no modern alternative?
Under iOS 11, those apps will no longer work. You may as well just delete them. And to help, there’s a spot in the Setting app where you can see a list of all those incompatible apps.