Process Explorer KPIs are values calculated based on formulas to display the Case Frequency, the Activity Frequency and the Throughput Time.
Click on the KPI icon in the upper left part of your process map, to open the available options:
By default, the "Case Frequency" is selected.
Custom KPIs
You might see more KPIs in this dropdown menu, that have not been described here.
There are Custom KPIs and have to be configured by an Analyst.