All UK visa applicants, save for those benefiting from a limited number of exemptions and exceptions, are required to provide biometric data (10-digit fingerscans and a digital photograph) as part of the application process.
You will have to go to your nearest visa application centre in person to provide your biometrics. In those countries where there is no visa application centre, you will need to go to the British mission. In the USA you are most often directed to you local Police Station.
Your visa application will not be processed until you have provided the necessary biometric information. The finger scans are electronic so staff do not need to use any ink, liquid or chemicals. You will have your digital photograph taken at the same time and the whole procedure should take no more than five minutes to complete. You should make sure that you do not have any decoration (such as henna), or any cuts or other markings on your fingertips before having your finger scans. You should also make sure that if you have any cuts and bruises on your face, they have healed or disappeared before you have your photograph taken. Digital photographs must be of your full face and you should not wear sunglasses, a hat or any other head covering (unless you wear it for cultural or religious reasons but the face must not be covered).