There are too many websites that have been built out of the desire to impress the CEO rather than the customers. Many websites are still very hard to navigate, most people have had at least one experience where they feel like they are banging their head against a brick wall, just to find one simple piece of information. People in this situation leave the offending website in 5 seconds or less and we think this is a ridiculous outcome from a website you paid good money for. We think websites have to be beautiful, but they also have to be simple and easy to use.

Usability is a fancy buzz word for making stuff that is intuitive and works the way people think it will. A real-life example is your common remote control. Most people want to click 'play', 'stop' and 'pause', but these buttons are normally hidden amoung tens of other buttons that make it very difficult for you to use the remote.
In websites, usability applies to many things, here are a few simple ones:

In the past, people designed websites that looked really nice for the first 2 seconds, then became very frustrating. Designers designed to impress other designers, not the target audience of the website - which meant a whole lot of happy web designers and a whole lot of useless websites. CEO's soon joined in on the action and wanted websites that reflected the way they saw their company, which was all fine and good except that their customers didn't see their company the way they did and once again, ended up confused.
We think that the people who matter most when designing a website are the people who use it, and these people are your customers. This means, before we start a website we consider: