Markup is a type of computer language that is used to indicate the structure of a document. i.e. how the information within that document relates to itself and other information.
Markup, specifically HTML, is the core of any web page on any web site. The markup tells the browser what pieces of the content mean and how to display them to your site visitors.
Proper markup will help search engines index your site correctly, which will help internet users to find your site or your business. Correct markup will also help browsers to load and display your site more quickly. In addition, proper markup will help disabled users to access your web site and get your information so they can be your customers as well!