What exactly is advertising revenue?

[Jake]

Most commonly that would fees charged for space rented for other to advertise their product or service, whether that is a billboard space in a front yard near a highway or a 720x90 pixel banner space at the top of a web site, in the case of web space ads, your role would be as the publisher.

The latter case "when google or facebook has adverts on there site to pay for the cost" is probably right, though "advertising revenue" is a bit vague, someone profiting from the use of paid advertising, like your first thought is probably earning Marketing Revenue