Unions are basically just groups of workers who have come together to try and collectively win better wages, health care benefits, job security, safe and respectful workplaces and fair scheduling. They are predicated on the idea that we can do better when we come together as a team, rather than going hat in hand to the employer as an individual.
If you are new to the United Steelworkers, here is an introductory video that we highly recommend: United Steelworkers - For a Better Life