Accessibility refers to making websites usable for those with varying levels of disability. These are often people with sight impairment which can range from colour blindness or eyestrain to a total inability to see.
The sight impaired visitor will often be using assistive technologies such as a screen reader, which will read the content to them. They don't have access to all the stylistic information that normally cues visitors as to what is happening on a page.
Accessibility is not difficult at all, and it is possible to implement without changing the style of your site in any way, provided you are not using a purely flash or image based site, in which case you can easily provide alternative resources in a text format.
From a business perspective, there are a growing number of sight impaired people on the web who won't be able to buy from your site without accessibility consideration.
It is also polite. Most of the things to be implemented are actually a standard part of good webdesign anyway, but it's always worth checking things over. A site that is easily accessible to the sight impaired is also easily accessible to search engine bots, so there really isn't any reason not to.
The main things to remember about accessibility are that all non text elements, including including images, graphical representations of text, image map regions, animations, applets and programmatic objects and flash objects, must have text alternatives. Many scripts will need no-script options and mouse-over actions will need to be usable with a keyboard.
The screen reader will read from the top of the html page to the bottom, so make sure that repetitive elements like navigation lists are at the bottom of the html page. CSS can style them to be at top when viewed by the average visitor, or anywhere else on the page, so that's not going to affect how the site looks. Likewise try to write the html code so that content is logically consistent when viewed without CSS styling.
Do not rely only on colour to convey information. About 8.5% of people have some form of colour blindness. Make sure that if you are using colour as a cue, for example with navigation items, that the colours contrast strongly in brightness as well as colour. There are online tools available to see what webpages look like with various forms of colour blindness.
Avoid excessive use of frames or tables. CSS make frames and tables much less necessary than they used to be. Frames and tables are no longer the only way to arrange a page layout and in fact they are not recommended for quite a number of reasons aside from accessibility. Screen readers do not see a series of frames or tables as laid out sequentially the way they look on the page, and using too many is confusing, jumbled and make content a nightmare to access.
When using frames, make sure there is a frame title, and make sure that table columns and rows have appropriate labelling.