Semantic in this case means meaningful. *g* Semantics is the study of the meanings of words, phrases, texts. When web developers talk about "semantic markup", we mean using tags in a consistent way to give meaning to web pages. So h1 has a specific, particular meaning (of 'first heading') and we always use it in that way, just like words have meanings.

If we used words just because they sounded nice, or similar to the word we really meant, or completely without reason, it would be even harder to understand each other than it is already. The internet for a long time has been coded in this way, and it has caused a lot of problems. That is what pne is talking about, I would guess; for web developers it is a very frustrating situation.