Bogdan is a designer and editor at DesignYourWay. He's reading design books the same way a hamster eats carrots, and talks all the time about trends, best practices and design principles.
To say things bluntly, usability is when you make your website easy enough, so visitors can find everything they need, when they need it, as easy as possible. A common opinion amongst web companies is that usability is pretty expensive to implement.