Your union contract is a written agreement with your employer. It defines your wages, benefits, conditions of employment, and rights on the job. It is enforceable through a grievance procedure and ultimately in a court of law.
Most union contracts are renegotiated every three years, although some contracts run for only one year and others run for as long as six years. How long the contract runs is up to you and your negotiating committee. The ILWU is a democratic union and members are involved in every step of the negotiation process.