Frappe is the framework. Over this you can install apps.
Until v14, ERPNext used to be a “monolite”, with all modules “inside”.
Now, ERPNext contains “main” ERP modules (accounts, sales, stock …) . “Satellite” things like HRMS, Payments, Healthcare, must be installed over ERPNext as apps. But there are many apps “outside” ERPNext like FrappeDesk, Gameplan … that just only needs frappe to get working.
This video will help you to understand it.
Hope this helps.