What does "clean, semantic HTML code" look like in the real world?

Many developers talk about the importance of "clean code" and "semantic HTML," but I can't help noticing that they do so without defining what those things are supposed to look like in a real-world project. For a dumb noob like me, it seems to me like everyone just assumes that everyone knows all the basics, and it annoys me.

What do they mean when they say "clean code?" Is it about formatting (tabbing, spacing, etc.)? Is it about logic? If it's about code logic, do you have any examples of comparisons? And what about "semantic HTML?" Are they talking about HTML tags, or do they include class and ID names in the mix? I would love to see some specific examples if possible.