A Highrise ID is a unique string of numbers that identifies every contact in a Highrise account.
When you update a group of contacts, Highrise will identify the contacts you're updating by Highrise ID.
The Highrise ID is one of the fields that will map during an import:
How do you find a contact’s Highrise ID?
When you export Highrise contacts, the first column exported is the contact's unique Highrise ID.