A biometric is a safety feature based on using “something you are” to confirm your identity. Fingerprints, hand prints and iris scans are all examples of a biometric. All in all, biometrics are a pretty good way of making sure someone is who they claim to be.
The international agreements on electronic passports require some form of biometric. In the case of the U.S. epassport, a digital photograph serves as the biometric, the same photo that is on the printed data page. That is a great security feature, because even if someone replaces a photo in the passport book, they cannot change the digital photo. The digital photograph is protected by an electronic “seal” that is broken if anyone alters the information stored on the chip. By comparing the person, the printed photo and the digital photo stored on the chip, border guards can quickly see if someone is using a fraudulent passport. (See, What happens at border control?)