This would create an event in Piwik with the category "Button Clicks" and the action "Submit Form". You could then use the Piwik dashboard to view how many users clicked on the button and when they did so. The Piwik postEvent function is part of the Piwik PHP Tracking Client library, which is included with the main Piwik installation. This library provides a range of functions for tracking data and sending it to your Piwik server for analysis.