Semantic markup is a way of writing and structuring your HTML (Hypertext Markup Language) to reinforce the semantics or meaning of the content rather than its appearance. In other words, HTML only focuses on the structure of the web page, while CSS focuses on the style of the content.

Semantic markup can help humans(without disabilities) and different assistive applications and devices(such as text-to-speech translators, text size controls, colour and contrast controls, e.t.c), understand the content structure, the relationship among page elements, and the nature of content inside an element and interpret these to people with disabilities.

It can also help improve SEO, code maintainability and quality.