The meaning of the word semantic translates to 'meaning' so, to basically summarize it, Semantic Markup is having meaningful tags. Its having the proper tags in the right places to provide for ease of reading the HTML.
Section 508 of the Rehabilitation Act of 1973 is a law that requires federal agencies to be aware of disabilities and make software and websites accessible to all so they can use it with ease.
SEO which stands for Search Engine Optimization, is used to increase the amount of visitors to the website. Using "white hat" practices, SEO aims to make the chances of a website being seen by the search engine higher.
How do They Relate?
When all these terms are put together, it will alow a website to be accessed and viewed by everyone, so it can be read by all including those with disabilities, and have a fair chance of being found due to SEO.