You really have to design from the start with it in mind. It's as the name suggests, to do with 'accessibility' for people with disabilities (visually impaired people for example), and will enable certain software to be able to 'read' the web page.

It can be a lot of work, but there's also simple things that can be done, like using descriptive alt tags for images, table headers etc.

Another way is to build a seperate, accessible version of a site (which are usually mostly text based, and not quite as visually attractive, but DO allow easy access to the information on a site)

A quick read of the following headings will give you a good overall idea of the sort of things you can do to make web pages more accessable...

http://www.w3.org/TR/WCAG20-GENERAL/

It can be a lot of work, but think of it as similar to legislation regarding ramps etc. for access to shops, it is a legal requirement in some places.