Records in custom lookup are searchable by Object Name or AutoNumber. By using the enhanced lookup in salesforce we can make a searchable record from many fields like name, phone, email, etc.
To enable it just follow the steps :
Click on Setup and Enter “Search setting” in the Quick Find box.
Click on Search Setting, and in Lookup Setting, check the checkbox of Object for which you want to build this functionality. If your object is not there it means salesforce did not provide the enhanced lookup functionality for that Object.
Click on the Save button.
Enhanced lookup feature is available only for some Objects that are Account, Contact, Custom Object, Opportunity, user, etc. Admin user has to enable this functionality for each Object.