It means the ease with which users can perform the tasks associated with your web site. Think of it as the quality of a user's experience when interacting with a product or system. When we're developing or creating a website, we try to keep things simple for your clients/prospects, if we keep it simple for them, then it will become a good website and it will invite people to contact you. Simple doesn't mean low quality, it means doing all that is possible to make it better and understandable.