CSF, KPI, metric

Answer:
CSF or Critical Success Factor is something that must happen if an IT service, process, plan, project or other activity is to succeed.
Key Performance Indicator or KPI are used to measure the achievement of critical success factors. With KPI you measure the achievement of CSF's.
Metric is, by definition, something that is measured and reported to help manage a process, IT service or activity. So this is direct item that you measure.

For example, CSF is: Maintain quality of IT services.
KPI's are: Total numbers of incidents, Size of current incident backlog for each IT service, Number and percentage of major incidents for each IT service... If you put KPI's in time constraint - you'll get trend. E.g. in last quarter or in last twelve months.

Se the article "Facing reality – measurements in ITIL" https://advisera.com/20000academy/blog/2013/04/02/facing-reality-measurements-itil/ to learn more.