You are an Authorized user when someone with a credit card adds you as an Authorized User. You will have your Social Security Number and identity added to their credit card. This will also give you the option of using the credit card you are an authorized user on.
Becoming an authorized user will add another credit reference to your credit report. This is usually only an option with a very good friend or relative that trusts you. Explain to them about what you are trying to do. Show them that you are taking steps to get your financial world in order. Maybe even show them this article. Many people do not understand so will probably turn you down. Being an Authorized user will help early in your credit building process but long term will not be needed.