It is simply an ID check. Each time a particular passport is used, the system will (triggered by the machine readable data) retrieve the photograph and fingerprint data. This will be compared with the 'new' data each time the passport is used. Once biometric passports are in place, the data will also be compared with the data embedded in the passport. Any immigration or criminal records are linked to a passport number
Also, the system will allow the US to track people using multiple passports (and there are some legal reasons for having more than one). Finally, your details will be held across a passport renewal ('cos the new passport will have a new number and cannot currently be linked to the expired one).