system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do TV and radio stations know how many viewers/listeners they have?
gpt
They have what is known as a sample audience. A group of volunteers or sometimes paid people from a wide selection of areas that have a device tracking what channels and when they watch on television. 
This sample is then used as an indicator of views, and an estimate is made  to give the overall number of viewers for any show.
They want to know this information primarily so they can charge their advertisers fees based on the viewing audience. Nielson Media Research is a company most commonly doing this, so you may have heard the term 'Neilson rating' before.