Lookup Filter

This restrict the values and lookup dialog results for lookup, master-detail, and hierarchical relationship fields.

This filter the records available in the lookup. This can reference other fields on the same record (source); fields on the records of the lookup object (target); fields on the user's record, profile, and role; and fields on records directly related to the target object.

Reference: https://trailhead.salesforce.com/en/content/learn/projects/customize-a-salesforce-object/create-lookup-filters

Visual: https://www.youtube.com/watch?v=UGwv8-ui8AA