HTML stands for
Hyper
Text
Markup
Language
It’s the code that is used to tell websites how to format your text. For example, to make something bold, you would put that text between tags that say <bold> and </bold>.
These are the fundamental building blocks of every website (including WordPress). Fortunately, you don’t need to know HTML to make or edit websites. But it can be extremely helpful (especially when you’re trying to tidy up weird formatting errors that always seem to find their way into your website.
If you use WordPress, you can see your HTML if you select “Edit in HTML” or “Code.” You can edit this and save it just like regular text. For best results, if you want to edit the HTML, copy and paste it into a text editor (like Notepad) to make your changes, then paste it back to preview them. This way if you aren’t happy with the changes you can just cancel instead of save. And even if you mess something up and save it, you can always restore the previous draft of a page or post.
Learning HTML is like learning an extremely easy language that can save you some headaches (depending how much you care about the layout of your website). If that sounds satisfying to you, you can find endless tutorials online.
CSS stands for
Cascading
Style
Sheets
These are the templates where the terms in your HTML are defined. For example, <h2> (aka Heading 2) might be 16 pt font, Playfair Display Bold, and CSS is where you would state that. (And so on for the innumerable terms within HTML.) In my experience, CSS is a bit trickier to teach yourself, but you can find tutorials for those too.
If you use WordPress, you will define these things using settings and graphic interfaces that update the CSS behind the scenes. If you want, you can see your CSS under Appearance > Theme File Editor. But DO NOT TOUCH it unless you absolutely know what you are doing!
Many, many people have taught themselves programming languages using online tools and trial and error. It’s not hard to learn a little of it, and it can help in ways you may not anticipate.