This neo-app.json is necessary for WebIDE to launch the ui5 application, and it will be automatically created by WebIDE if you create a new project there - normally we do not need to change it manually. However, if you clone a ui5 project which is not created by WebIDE ( for example created by Eclipse ), then you have to add this neo-app.json manually.
For entryPath you can just keep its default value, and the mapping for it could be customized, as displayed in the example below:
