What characterises an inherence factor ("something you are")?

It relies on a biometric characteristic of the subject's body, such as the iris, retina, facial features, fingerprints or the vein pattern in the hand or fingers.

A biometric sensor measures the characteristic and compares the sample with a stored reference (the template). The result is a similarity score, not an exact match: the system accepts if the score passes a threshold. That makes biometrics fundamentally different from secrets:

• Probabilistic: there is always a trade-off between wrongly accepting someone else (false match) and wrongly rejecting the owner (false non-match), set by the threshold.
• Convenient: nothing to remember or carry, which is why fingerprint and face unlock are so widespread.
• Bound to the body: it cannot be lent like a password, which is exactly what makes it attractive for proving presence.

In practice biometrics usually unlock a secret held on a device (a phone's key store, a smartcard) rather than being sent to a server. Used that way, they act as the activation factor of a multi-factor authenticator.

Go deeper:

• Biometrics (Wikipedia) — biometric functionality, performance measures and their trade-offs.
• NIST SP 800-63B-4: Use of biometrics — accuracy, presentation attack detection and the rule that biometrics need a physical authenticator.