There are certain characters in HTML that are reserved by the language, for example < and >. Using them in an HTML file for something other than tags may result in errors because your browser may interpret them as tags. One way to get around this issue is using HTML Entities:

< can be written as < while > can be written as > and they will render as expected in the browser.

A full listing of HTML entities for other reserved characters and characters not in your keyboard such as © and ∑ can be found at https://dev.w3.org/html5/html-author/charref.