This is an acronym for HyperText Markup Language. HTML is a simple programming language used to display information in a web browser. Without HTML you could not view a web page.
It is a simple language to learn. Although there are several hundred commands, the typical website owner only needs to learn a few dozen to manage their website.
The content management system you are using should handle most, if not all of your individual programming needs. But you will need to add a few basic HTML commands if you really want to spice up the usability and friendliness of your website.
Again, it's not difficult to learn a handful of HTML commands. Every person I've trained has felt a sense of accomplishment as they learn to use HTML codes.