That's kind of a complicated question. There's a few pieces of information that can be helpful to tackle it. One is that I once came across a series on Netflix which for the language had Swiss German (or possibly just German and I uncovered the variety by myself). I couldn't understand it at all.

Then, you probably remember how I said that Dutch could be an acceptable middle ground between English and German. Well, I recently realized that there's even more steps, one of which is West Frisian. The phenomenon is called a "dialect continuum" in linguistics. If you moved from English to West Frisian, to Dutch, then to German, you'd find that you could understand each new language without much study after the previous one. And the transition is more or less geographic (English is to the very west, German is to the very east).

Except, there's gradations like that within Germany (and neighboring countries). I guess you'd call them separate languages. Unfortunately their speakers weren't inventive with naming (or rather I suspect there's a complicated history of politics and conquest involved). And so they are all "German" with an adjective. Or "Deutsch", which according to Wikipedia goes back to a common Germanic root which means simply "people".

Of course, it doesn't become a problem usually until pretty deep into the study of Germany and its culture. There you begin trying to specify "Hochdeutsch" for Germany's lingua franca. High German as the language is a little bit artificial and was in a sense crafted for this purpose in higher society (although that's not why it's called that). I believe it's High German because it's a distinguished member of the High German family of dialects, which is called that because it used to be spoken at higher elevations, closer to the Alps. Whereas the Low German family of dialects used to belong to the lowlands, along the coastline.

Hopefully, this helps somewhat, although I can't vouch for 100% accuracy, because I myself still struggle with this topic (and, I suppose, even professional linguists do). Of course, the are Wikipedia articles, but they could be overwhelming at this stage.

Another detail, I can somewhat understand Low German, without studying it, on the basis of my familiarity with High German... Although people speaking High German kind of like to rely on it to add color to their speech and for slang, so maybe I'm more prepared than I could be.