The webapps.counter vocabulary defines a subclass of dispatcher:

TUPLE: counter-app < dispatcher ;

The <counter-app> constructor word creates a new instance of the counter-app class, and adds a page-action instance to the dispatcher. This page-action has its template slot set as follows,

{ counter-app "counter" } >>template

This means the action will serve the Chloe template located at resource:extra/webapps/counter/counter.xml upon receiving a GET request.