Cobrowsing comes in different forms and sizes. Some providers enable you to highlight areas on your visitor's screen. That makes it a lot easier to answer your customer's questions by directly showing them the elements right on their screen. Cobrowsing is comparable to screensharing but it is limited to the browser window.
The operator usually has to ask the visitor for permission to enable cobrowsing first. You can even take control over your visitor's mouse and browse to another web page. This can be very handy, especially when your support deals a lot with inexperienced users.