In web design, accessibility refers to how well people with various disabilities can navigate a page. For example, a person who is hearing impaired would be able to read a site without difficulty, but any features that include sound or spoken narrative would be useless for that user. People who are visually impaired often use special software that will read a page out loud to the user. Screen reading software is able to read all the text on a page, but it may or may not be able to process images, tables or animations.
The Web Accessibility Initiative is a non-profit organization that researches, identifies and disseminates information about design elements and practices that can make the web accessible to as many people as possible. Their website is an excellent place to go for exhaustive information on how to make your site accessible.