The key value that provides an additional level of sorting and/or aggregation in your report.
Many report tables display a row for each possible combination of primary and secondary dimension values. For example, if you select "City" as the primary dimension and "User type" as the secondary dimension, and data exists for New York, Lima, and Berlin, the table will have six rows: New York, new users; New York, returning users; Lima, new users; Lima, returning users; Berlin, new users; Berlin, returning users.