Semantics means knowing and interpreting the meaning of words,---or, more fundamentally, what a given person in a given instance means by a given word or words.
So, when a baby says "Bottle" in asking for his bottle, you would be making a semantic error if you thought he means he wants every bottle that's on the planet just because he does not specify that he wants only his.
A common phrase about semantics is "It's all a matter of semantics", meaning that it comes down to what one thinks the words mean.
Semantics is part of classical, Socratic, logic, in that the idea of formal semantics is the same thing as the formal, that is, reductive, use of classical logic. For example, the reductive sense of the statement, "I got four hours of sleep last year" means that no more nor less than that "I got no more nor less than four hours of sleep last year."
Of course, for sake of natural efficiency in everyday speaking, we normally make use of implied context. So, by the statement "I got four hours of sleep last year", we normally mean "I got four hours of sleep last year in contrast to getting plenty of sleep last year.".
But, in the non-reductive (but still literal) sense, the statement "I got four hours of sleep last year", can mean any of a host of things, including something along the lines of "See those two horses?" or "Look at the poison oak rash on my arm!"(while your legs, too, are covered in the rash): "I got a certain four hours of sleep last year, and I'm not concerned with mentioning all of the other hours of sleep I got last year."