The Key Performance Indicator is industry jargon for a type of performance measurement.
KPIs are commonly used by an organization to evaluate its success or the success of a particular activity in which it is engaged [source: Wikipedia].
It is used to indicate the different stats of the elements in ClickMeter account. Users can get a quick overview of the count of tracking elements/tracking pixels, clicks/views, number of guests, last clicks etc.
More about ClickMeter KPI reports here