Scoop uses the term App to refer to an application or program to be installed. You might also see it used to loosely refer to a manifest that describes how to install an app.
An app is a self-contained, independent unit. It contains one or more executables or scripts. But that's probably too much of an explanation—generally you'll know an app when you see it.
Probably the main reason for the term 'app' is that it only takes 3 keystrokes on 2 keys to type.