The value of a primary key in one table has a matching value in the corresponding foreign key of another table.

Referential integrity means that relationships between the tables must always be consistent.  Any foreign key must agree with the primary key that it is linked to.

Any changes to a primary key must be applied to all linked foreign keys.

If a Pilot ID was changed in the Pilots table, then any records in the Flight table that have the Pilot ID as a foreign key would also be changed