What is the event_id for?

It is a reference to the audit_event table which contains extended information where the audited event happened.

How does it work?  

It ties an audit record back to teh audit_events table.

The audit tables are extended by an additional column event_id which points to the event that initiated the changes which are audited:

For this event a record is saved in audit_events which describes in the column source the reason for the data changes in a JSON structure.

The column parent_id in both tables points the the record which was changed.