HTML stands for Hypertext Markup Language. When browser software accesses any file it understands how to parse the document according to the file's extension(.html - .txt - .php - .xml - .pdf - .doc - etc... ). HTML provides us a means of laying out and structuring our web pages using paragraphs, graphics, images, audio/video media, applications, forms, user interaction and much more.
Browser software interprets exactly how to render the document by the HTML elements laid out in it. An HTML document is just a well dressed text(.txt) file, given a different extension(.html) so that browser software can process it properly. It is important to know all of the elements at your disposal, this way you can choose the correct elements and avoid using elements out of the context for which they are specified.
HTML elements wrapped around content making it render the way the author wants:
<h2>Movie List</h2> <ol> <li>Star Wars</li> <li>Avatar</li> <li>Interstellar</li> </ol>