The Metric is a core Blueprint concept that allows users to link projects, processes, applications and activities to key performance indicators that are of interest to the business. The Metric is defined as a qualitative or quantitative number that allows a business to rate it's effectiveness in a given area. It helps answers the question "Why?" Why are we performing this function and what affect do we expect it to have on the business? Process Tempo concepts use the "Influences" relationship to define how a part of the business impacts a given metric.
A good metric example :)