What's the deal with the biometrics fee and appointment?
Many immigration processes, including the processes involving getting a green card or applying for citizenship require that the applicant is fingerprinted. The reason why the applicant needs to get fingerprinted is because an FBI criminal background check is going to be run to make sure that the applicant doesn't have any criminal history that would bar them from getting their green card, citizenship or other immigration benefit. Basically, they are looking for criminal issues that would make the applicant deportable or inadmissible. The applicant has to submit a fee for "biometrics" at the time that they apply for their green card/naturalization/other immigration benefit and then get their fingerprints taken at their local (USCIS office or Application Support Center (ASC). The applicant cannot just get their fingerprints taken at the local police station and then submit it to USCIS, it must be done through one of USCIS's Application Support Centers!
What happens at the biometrics Appointment?
The applicant needs to bring their appointment notice and a valid photo ID to their biometrics appointment. At the biometrics appointment, the applicant will get their fingerprints taken and sometimes they will get their photo taken as well. The appointment doesn't take very long. The fingerprints are then sent to the FBI to review, and USCIS will also check the fingerprints against their own records.