Key Performance Indicators are Metrics to measure the status quo.
What gets measured gets managed. In the world of business, and especially in the realm of manufacturing, Key Performance Indicators (KPIs) are used as to measure (and in some unfortunate cases, define) success. They measure the status quo. Ongoing operations have KPIs, specific initiatives have OKRs. Both are forms of SMART Goals.
KPIs are lagging measures of output that can, in theory, surface small problems before they become bigger ones. (So both leading and lagging depending on viewpoint).
A KPI measure has to have a clear & unambiguous definition, should be associated with a grading scale, and should be part of a company’s Periodic Reviews.
KPIs are not without faults. See Gaming KPIs