Usability is about ensuring that when users access your website they can find the information they are looking for quickly and efficiently.
- Your website must be easy to navigate
- Pages should download quickly
- Information should be easy to retrieve
- No restrictions should be placed on users
You can find a good introduction to usability in the article Web usability - The basics on the Webcredible web site, a great source of information on usability and accessibility. You can also read a number of other articles about usability on the same site and download a white paper. (You'll need to provide an e-mail address to have the link mailed to you.)