Online visibility is making yourself widely available to anyone looking for you online what you look like when they find you. The very process of being widely available is self-reinforcing. If the only thing you have online is a website then Google will not consider you to be very important and you will find it hard to rank. But if you also are listed in places like Maps, Directories, Review Sites, Industry Niche Sites, Local Sites, Social Media Sites, Video Sites then suddenly the likes of Google sit up and take notice. This helps your website to rank better.
One of the problems with being widely available is to ensure brand consistency. You may put a lot of effort into getting your website just right in terms of look and feel, but what happens if someone finds you in one of the other places listed above? Has anyone made sure that you will be recognised as the same company as the one on your website? That is an important part of your online visibility and it has a bearing on your online credibility too.