What are deleting apps?

No idea what you are asking.

Please explain

There are some free and some paid, but for mac osx: appdelete, appzapper, appcleaner and so on.

In general it is best when you use the uninstaller that comes with a good app.

No such thing in iOS: you delete the app by holding it, then click the little black x. This dletes the app and its user data.