An app's developer can retire it when they stop offering it. Retiring doesn't remove the app from your store: you can keep using it for as long as you like.
What stays the same
The app keeps working as before, and you can still open it from Apps.
Your data stays as it is, both in your store and what the app keeps for you.
Billing stays as it is: a subscription or charges you agreed to continue as before until you uninstall the app.
What changes
The app gets no more updates.
It no longer appears in the App Store or in app search, and no other store can install it.
If you uninstall it, you can't install it again.
Uninstalling a retired app
Uninstalling works the same as for any other app, and Tringify shows what it will change on your store before you confirm. See Stop an app's service or remove it. Because you can't reinstall a retired app, find a replacement first if you still need what it does.