This is a simple app template for Walnut - unlike the example within the Walnut repository, this keeps Walnut as an external submodule and is much more sensible for actually building applications. See the Walnut repository for more details.
Once you've cloned, you can customize the premake5.lua and WalnutApp/premake5.lua files to your liking (eg. change the name from "WalnutApp" to something else). Once you're happy, run scripts/Setup.bat to generate Visual Studio 2022 solution/project files. Your app is located in the WalnutApp/ directory, which some basic example code to get you going in WalnutApp/src/WalnutApp.cpp. I recommend modifying that WalnutApp project to create your own application, as everything should be setup and ready to go.