Semantic markup refers to the use of HTML tags and other markup languages to provide additional meaning and context to content on a webpage. This markup helps search engines better understand the content of a webpage and can improve search engine rankings. It also allows for more efficient and accurate indexing of the content, as well as better accessibility for users with disabilities. Semantic markup can include tags such as <header>, <footer>, <article>, and <nav>, among others, to provide structural and contextual information about the content on a webpage. Overall, semantic markup helps to improve the overall user experience, search engine optimization, and accessibility of a website.