You should have received a copy of the collective bargaining agreement when you were hired. This specific provision was ratified in a special election of all faculty, union members and nonmembers, full-time and hourly. You have become an Agency Fee Payer, perhaps by default.
Members receive all the privileges of membership, including the right to vote and hold union office.
Most faculty, including a majority of both full-time and hourly faculty, are members. Very few of your colleagues have chosen to refrain from joining the Association. To become a member, you must fill out a membership form. If you do not fill out a membership form you become an agency fee payer by default. Agency fee payers may not vote or hold office, but they do receive all other benefits of membership.