This is the heart of the web. The basic definition that gives structure to your web pages. Having a good understanding of HTML will help you regardless of which frontend framework you choose for developing your web app. Understanding the DOM is paramount for understanding anything related to frontend frameworks.
Don't spend too much time on this as we'll likely use custom components in angular or react.
We recommend that you brush up on your HTML5 fundamentals over here.
The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the web.
We strongly advise you to clarify your grasp of DOM. Please read this documentation for more information.