If something is semantic, it has to do with the meaning of a word. If you're spending all this time reading the dictionary, you must be interested in semantic questions — or you just want better grades on your vocabulary quizzes.
Semantic comes from the Greek word for "significant," and has to do with how, say, the word dog actually means that furry friend of yours, and all the others like him. If you're really into the philosophy of language and how words come to have particular meanings, then you like semantics. It can be an adjective, as in a semantic argument with your mom over the meaning of "grounded," or a noun, meaning "the study of signs and meaning."