Haha same here, I’m a designer, but learned quite a lot working with Webflow (tutorials, reading up on stuff, etc…). There is a ton of cool videos that help you learn all that.
CSS is basically any styling added to your elements (size, color, opacity, position, etc…). If you have zero idea about how to edit the files, I recommend just starting with the basics on https://www.w3schools.com. It’s an easy way to understand the difference between html and CSS functions (CSS is basically a property that defines elements’ styles, across all pages (html pages), usually found in a linked CSS sheet. When a CSS is found inside the HTML file, that’s when it’s an inline-css, and can sometimes slow down load speed).
I also recommend you run your website by this https://web.dev/measure/
It’s a free tool that will give you a detailed report on what to do to further optimize your site. They also link to outside articles that guide to what you need to do.
Hope this helps!