If anyone has ever listened to you counter-argue a point you have made only to cut you off with the cliche, “Oh, well, that’s just semantics!” they are absolutely correct. They are also, technically, not being very specific in their critique. What they probably mean by “just semantics” is that you are saying pretty much the same thing they did, but in a slightly different way. What they may not be considering is that how you say something ultimately affects the meaning.
Semantics is a field that deals with the study of linguistic signifiers and “signifieds”; in other words, semanticists are interested in the ways that people use words, clauses, sentences, paragraphs, and discourse to create meaning and communicate. So, yes, it is just semantics, but semantics can refer to any and every kind of work humans attempt to perform with language.
Semantics is a sub-field of semiotics, or the study of signs and symbols.