A trade union is a group of workers who come together to support each other and negotiate with their employers. They work together to improve their working conditions, salaries, and benefits.
In a more complex context, one might say:
In general usage, "trade union" specifically refers to labor organizations; however, it can also be seen in contexts related to politics or economics when discussing workers' rights or labor movements.
While "trade union" does not have specific phrasal verbs associated with it, you might hear:
A trade union is an important organization for workers, helping them to work together to make their jobs better.