Metrics are quantitative measurements. They are used to assess the status and efficiency of a particular process, plan or product like an advertising campaign, Landing page, etc. Metrics allow organizations to evaluate if they are meeting their goals or falling short.

List of Piwik PRO metrics can be found here:
https://help.piwik.pro/support/custom-reports/custom-reports-metrics/#basemetrics

42 KPI Ideas for Your Product Analytics