Block Editor is where you program for apps logic and its behavior. In this editor, you’ll see all the components that were added in the design editor. The block editor provides you a drag and drop interface to program the behavior of the app by piecing together jigsaw-like blocks.
In design editor you visually select components and layout the apps screens layout. In block editor, those components will be available for programming. Each component type (e.g. Button), will have a set of pre-defined properties and event handlers which are presented in form of blocks.