Before you start to work with Harmony, you need to know and understand some concepts. Also it's important to have some background related with the platform. One of the greatest things of Harmony is that it allows to switch beetwen platforms very easy because the concepts are the same and the implementation is similar.
Harmony is based on clean architecture. Clean architecture has 3 different layers: application, domain and data. Each layer has one responsability. Also, they are connected to each other but got boundaries. https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html
Application or presentation layer will include all views and also specific platform stuff.
Domain layer will include all business logic and interact between Data and Presentation layer by means of interface and interactors.
Data layer will include POJOs and means to get Data from cloud or local storage.