What is a “metric” in Google Analytics?
A segment of data separated out in a report for comparison.
The dates in your date range.
A dimension that can help you analyze site performance.
The numbers in a data set often paired with dimensions.
Metrics are quantitative measurements. The metric Sessions is the total number of sessions. The metric Pages/Session is the average number of pages viewed per session.
Read more here: https://support.google.com/analytics/answer/1033861