In Composer, a metric is a numeric or attribute field that has been aggregated. Numeric fields can be aggregated using aggregation methods SUM, AVG, MIN, MAX, and LAST VALUE. An attribute field can only be aggregated if its data source configuration definition has the Distinct Count option enabled. See Enable Distinct Counts. See also Metric Aggregation Functions.
After a numeric or attribute field becomes a metric by aggregation, it can be used in the y-axis of visuals (that support a y-axis). The aggregation of a metric can also be changed in tables using the table context menu and the Table Settings sidebar. See Change Metric Aggregation in Tables.