They simply equate strings of letters. HTML, the system of tags used to create web pages, describes presentation of material, but it tells nothing about the meaning of the text or images involved. The Semantic Web will change all of that. First consider that every object mentioned on the Web is a resource. Whether that object is you by name, your job title, a product for sale or a research project.
Meaning is described in sets of triplets, much like simple sentences.
Each triplet has what roughly equates to a subject, a verb and an object. Wendy is parent of Mike. Mike is student at Avondale University. Ontologies, a word co-opted by artificial intelligence and web researchers, are collections of information posted to the web. They describe relationships.