Dom is called as Document Object Model. DOM is a programming API for HTML & XML. 

DOM is a logical structure(tree) of the document. 

Shadow DOM is a standard that encapsulates the internal document object model (DOM) structure of a web component. 

Since not all browsers that Salesforce supports implement Shadow DOM, LWC uses a shadow DOM polyfill. A polyfill is code that allows a feature to work in a web browser.


Here's a simple example of using the DOM in LWC: