Making the web usable is about using the web standards to create web pages, websites and web applications that are accessible to people with a variety of physical or visual disabilities as well as the search engines, too.
It's also worth noting that accessibility isn't merely for those with disabilities – everyone benefits from good, clear structure and content.
More over, individuals and businesses making use of devices such as smart phones and hand-held PDA's also have a healthy appetite for content, such as news, sport, weather and entertainment.
These too rely heavily on solid, standards-compliant and accessible structure.
For those people with certain disabilities, such as visual and physical impairments, websites can be impenetrable barriers to the data & information that they're after, rather than being the fonts of knowledge that they are to others.
While often simple good practice is all that is needed, in some cases, more care is needed to deliver a product that is not only open to the various screen reading tools, but is also simple to navigate and explore.