The appointments generally take a very short amount of time. Generally you show your appointment notice and proof of identity. Then they fingerprint you and take your picture. They stamp and initial your appointment notice to prove that you complied. You get to take the notice with you when you leave.
You will need to have proof of identity, such as an unexpired passport or state identification card.
Your photo and fingerprints will be taken. You should get a receipt to prove that it was done. Unless, you have have an outstanding warrant for your arrest or detention, there is nothing more that you should expect.
The above is general information and does not create an attorney client relationship.
Typically, immigration biometrics are electronic fingerprint scans and a digital photograph of your face. It may also include facial recognition scans, iris scans, and a variety of other ways of measuring unique body characteristics for identification purposes.