Audi123 in a nutshell, apps can communicate with each other via "IPC" or inter-process communication. It's an important part of Android and many features rely on it, i.e. opening files with another app, share buttons, notifications, etc.
It's important to note that IPC only works if apps are set up to do so. Apps cannot just arbitrarily start sending data to other apps or request data from them.
Many in the community want control over whether apps can communicate with each other. An example would be Google/Pixel Camera. Many people want to use it but are afraid the app will communicate with Google Play, so they don't use it. With App Communication Scopes, we can isolate apps from others.