Blender Apps are an upcoming Blender feature that were recently discussed/proposed on the Blender development blog. In a nutshell, Blender Apps are a way to build new applications using Blender itself as a foundation. Instead of extending Blender like you do with plugins/add-ons, Blender Apps are new experiences that make use of Blender as both a runtime and as a framework.
The Blender App process builds on top of Application Templates feature released in Blender 2.8, which enabled you to create tools using a customized Blender UI. The Apps approach however is going to take this a few steps further, by introducing a new file extension Blendx, which can either be run using your local install of Blender 3.x, or bundled with the Blender application, to make a seamless executable.
Perhaps the best way to understand Blender Apps is to see a use case scenario:
“As a studio TD, I want a cross-platform solution that can visualize everything from image sequences to videos and 3D files, and annotate on top of such media so that I can give notes to colleagues.” Combining Blender’s Video Sequencer, Image Editor, Annotation tools, making a nice UI for it and stripping out the rest of the functionality you don’t need would be a good example of a Blender App. A version of this example (using Application Templates) is already in use at the Blender Studio weekly meetings.
“As an architect, I want to send a client a project so that they can navigate around and experience it by themselves.” In this case, the app should be easy to run even if the client has never heard of Blender or 3D software before. You could prepare the scene, add basic navigation controls, and bundle the project .blend files together with Blender itself. The client would receive a .zip file with a single executable file inside.
At this point Blender Apps are still in the proposal/development phase, although you can see the example Media Viewer (links below) that use the existing Application Template approach, which will be very similar to how Blender App development will occur.
You can learn more about the Blender Apps proposal and learn details on how applications are developed in the video below.