This metric returns the number of check constraints that have their is_not_trusted flag set to 1 in the sys.check_constraints table. Untrusted constraints force SQL Server to construct less efficient query plans, because it doesn’t know enough about the kind of data contained in the table. This can point to a data integrity issue which should be investigated.
If the metric and alert tells you that untrusted constraints exist, find out which ones they are by running:
SELECT * FROM sys.check_constraints WHERE is_not_trusted = 1 AND is_not_for_replication = 0
For details of how to fix them, see Brent Ozar’s article: Blitz Result: Foreign Keys or Check Constraints Not Trusted.Untrusted check constraints,