A page view is an event type. It is used to create the page views metric, which is the total count of how many times a page or group of pages had the page view event triggered.
In Piwik PRO, a page view is triggered by default by our tracker on each browser reload. There is also an option for single-page applications (SPA), where the tracker uses renders of different views to trigger different kinds of page views in an interactive application.
Read more about tracking page views in SPAs here: 3 ways to track single-page applications (SPAs) with Tag Manager: a step-by-step guide