This metric returns the number of foreign keys that have their is_not_trusted flag set to 1 in the sys.foreign_keys table.
A foreign key points to a primary key that must exist in another table, for example, column X in Table 1 must also be present in Table 2. The key protects this link, and only valid data can be inserted in the foreign key column. An untrusted foreign key may threaten a database’s referential and data integrity.
If the metric and alert tells you that untrusted foreign keys exist, find out which ones they are by running:
SELECT * FROM sys.foreign_keys 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 foreign keys,