What is a usable website? It's having a website act the way a user expects. It's having information available that's expected to be available and easily findable.
An extensive usability study may involve hiring a group of strangers who agree to be on video camera while they use the website. They are asked questions about where certain information is located on the site. The user then attempts to find the information. The reactions to the questions are measured and counted, such as the number of clicks it takes them to get the answer, or note the emotions and facial expressions as a measurement of frustration. The statistical data is analyzed to formulate usability patterns, and identify possible problem areas with the layout of information.
A simple usability study could involve asking your friend to find a specific piece of information on your website and watching them interact with the site. It's better to ask more than one person to do this because if a pattern evolves, there is a problem with the layout of your information.
This potluck sign-up is a good example of usability because: