Run of Network advertising is a form of marketing

Marketing

Marketing is the process used to determine what products or services may be of interest to customers, and the strategy to use in sales, communications and business development. It generates the strategy that underlies sales techniques, business communication, and business developments...

which lets an advertiser post an online advertisement (such as IAB Banners) on a wide collection of websites without the ability to choose specific sites. Networks represent the banner advertising on many sites and might be focused only on one type of site such as Games or Entertainment or have several different categories they represent. The categories are called Channels. Having a Run of Network option should result in lower prices than choosing sites specifically as the network can run on more sites and take advantage of extra inventory that might not be in as high demand. A good network should optimize Run of Network traffic so they are not showing ads to the wrong people. Some examples of advertising networks are:
Google has a very large network where every type of advertising category is well represented.
CPMStar is an example of a smaller network that focuses solely on one type of site. They represent over 500 game sites and cater to advertisers that are looking to reach gamers.