App Inventor has a neat featured called “live programming”. With this feature, after transferring your app from the App Inventor “cloud” designer and blocks editor, you can make changes in the Designer or Blocks editor while your program is connected and running. Try it! Create an app, transfer to your phone and start running the app – then go back to the App Inventor browser window and make some changes to your program. Moments later, your changes will show up on the phone!

Here is a bit of history about how this feature came to be, and how it was implemented: Colorless Green Ideas Sleep Furiously: The creation of live programming in App Inventor.

That summary is pretty techie and might be beyond someone very new to programming.