The reach of a broadcast medium (such as a radio or TV station) is the number of people who use it over a given period of time.
Reach is one of a number of statistics that are collected on the broadcast media, the other important numbers being total viewing/listening hours, viewing/listening hours per person (total hours ÷reach), and percentage share of listening hours.
Reach tells you how many people are interested enough in a broadcaster to at least tune in for a few minutes. The statistics usually only include people who listen or view for a minimum time. From an advertiser's point of view, it can be used to estimate how many people are likely to hear or see a broadcast ad that is regularly repeated. Higher reach means greater ad revenues. Listening hours and listening hours per person are equally important metrics.
Advertisers will also be interested in how closely the audience a broadcaster can deliver matches their target market. They will pay more for a more affluent or better targeted audience.