Metrics are another name for the statistics measuring quantities of such items as the number of unique visitors to a web page, or the number of new customers purchasing in a given time period, the number of clickthroughs in a day or week, ad impressions per hour, day, or week, etc. Metrics are typically the raw measurements of such items, as opposed to "massaged" numbers. Essentially it means "the number of things measured" or "the number of things metered" (as through a gas meter or speedometer).
Celebrating 25 Years of Creating an Island of Stability In the Sea of Online Confusion