Strictly speaking semantics is the study of meaning, but it has also come to mean adding tags and other ways of adding meaning to text or code. For example,
My Title is just text but if we add the HTML headers tags to it
<h1>My Title</h1> we give it a more precise semantic meaning as a level one heading.
The Semantic Web
The sematic web was an attempt to make all the content on the web have categories of meaning. The movement largely failed when the Web was hijacked as an application platform instead of its original purpose for documentation and knowledge transfer. The HTML elements of the semantic web persist and can be used by those who wise enough to draw a clear distinction between the very different uses of the Web Platform.