Semantic markup- Is necessary for basic web development. Semantic markup or semantic HTML is basically the use of HTML markup to reinforce information on your webpage. It can be used to create a header on your page or to create a paragraph and many more options.
Section 508 Accessibility- Section 508 is a federal law that states all technology used or made by the government be accessible to anyone with a disability. This means web developers must correctly set up their codes to include ways for the disabled to use the web page or they could suffer legal consequences.
SEO- Search engine optimization is the process of making sure a web page shows up in search results as a method to maximize the sites visitors. Payment it not involved and results from search engines are based purely off of what they believe to be most relevant.
How are they related? I suppose the theme would be accessibility. Without Semantic HTML Section 508 Accessibility regulated webpages would not be able to be created and without SEO people would not find accessible websites as easily.