First of all meaning of Semantic is, something that is related to meaning or logic, i want here to say that if something makes sense or pointing to something meaningful or logical.

for example in real life sometimes when you talking to someone and you suddenly say "iss shir" so here this makes no sense no meaning in any language and this is not logical what you just say.

But the ponit is, the person with whom you are talking about would understand what you say because he knows that you always talk in that accent, but it makes no sense in any language and this is not logical also so here this line "iss shir" is non sementic and so if you want to make it semantic you have to say "yes sir".

Now,

Same as in HTML with HTML tags,
Any tag which is meaningful and pointing to any specific
for example, header tag here represent that it is used for the purpose of header component of a webpage.

So this header tag makes sense its purpose and depict meaning-ful-ness that it is a type of tag which should used as header in websites webpages.

So here header is a semantic tag.

And there are almost hundreds of semantic tags in HTML
some examples other than header are article, nav, footer,
form .

And by keeping this point in mind most search engines use this for SEO.

Now I KNOW YOU ARE INTELLIGENT so curious also that how they use this as SEO(Search Engine Optimization).
Find it from your Own its your lets say homework.

Bye Hope you enjoy and learn something new from this post.