There are a ton of replies in here, including yours which seem to be trying to explain what this is, but appear to be assuming too much, and still skipping the actual explanation.
Drive out to a park. Set up a radio. Announce you are in a park. You are known as 'activating' the park. People in other places answer, they are known as 'chasers'.
Why? People like making radio contacts with people in novel places. People like setting up their radio in cool places.
There is some gameification, and points gained for activating and chasing. The points get you nothing, not everyone even records them.
