Semantics refers to the proper interpretation of the meaning of a word or sentence. When it comes to it's use in HTML tags they have to have a certain semantic meaning. Where the elements themselves have to convey some sort of information about the type of content contained between the opening and closing tags.
Web Content Accessibility Guidelines (WCAG)
Web Content Accessibility Guidelines is developed through the W3C process with the help and cooperation of organizations and people around the world, with a goal of proving a single shared standard for web content accessibility that meets the needs of individuals, organizations, and governments internationally.
Section 508 is an amendment to the United States Workforce Rehabilitaion act of 1973, it is a federal law mandating that all electronic and information technology developed, maintained or used by the Federal Goverment be accessible to those with disabilities. Websites that are properly created and designed, allows all to have equal access to information and functionality.
Search Engine Optimization
It the process of maximizing the amount of total visitors to a website. Search engines have been compared to the libraries of the internet, useful for looking up specific documents and general information. With search engine optimization it allows for links, works,and titles to have the best results and reputations when sought after.
When combined the result is an optimized experience for those that use the internet of all types. This allows for anyone to use the internet effectively and reliably. Given these standards and regulations that follow it ensures equal, fair, and accessible use for all users.