"Accessibility" is a broad term; it refers to how easily a disabled person can access something compared to the nondisabled. In the physical world, there are wheelchair ramps and disabled parking; for television, there's closed captioning and audio description for the blind.
Web accessibility is about making sure your users, no matter what they're using on your web site (whether it be through Lynx, a GUI browser with images off, or a screen reader) are able to access the content.
- Dive into accessibility - a bit dated (most of the "How to do it" sections are about wrangling weblog publishing tools into doing things they already do), but very useful.
- Building Accessible Websites - the book on web accessibility; at the very least, you can laugh at the cover's inadvertant, er, resemblance to an Internet icon.
- Building accessible websites serialization - A version of the book for the web, with large chunks of content stripped out.