Lead ID - What is it and what should it be assigned to during import? The mandatory field 'Lead ID' must be mapped.

Lead ID - What is it and what should it be assigned to during import? The mandatory field 'Lead ID' must be mapped.

Hi, I am importing my contacts from another CRM via csv file.  In the Data Migration section, it says the LEAD ID field is mandatory, but there is no information on what this should be mapped to?

Error message shown:  The mandatory field 'Lead ID' must be mapped.

What is LEAD ID?
What should it be mapped to during import, as I have no field that matches anything like that name?