What is accessibility?
People have an accessibility issue if they have a disability, either physical or mental that stops them using a normal computer to get online.
Perhaps the easiest example to think of is someone who is totally blind. When nearly all our information was from books Braille was invented to make blind peoples life's easier. If computers where simply visual devices we probably wouldn't have as many accessability issues as we do. Unfortunately computers are interactive they rely on sound and vision and so as we use them more making this use easy for people with disabilities becomes more and more important.
Perhaps the most important thing to say is if you as a web site designer follow some simple rules the web sites you make will be easy for the special equipment and software people with accessability issuesuse to access and they will get as much from the web as the rest of us.
If you think how difficult it has been to make the physical world accessible to people in wheelchairs after we have built all the buildings rather than doing it as we built the buildings. You will hopefully appreciate we should build an accessible Internet now and not fix it later.