a union is a legally protected way for works to collectively bargain with their employers. Internally, union dues are used to pay for legal assistance (among other things) that can help the workers. A union can make the affiliated company do things by creating legal agreements and organizing strikes. A company often finds it is easier to give into a unions demands rather than have their employees stop working and hold signs outside letting everyone know that they are being treated poorly. A company can tell a union to fuck off and they often do, this leads to the workers going on strike and bringing business to a standstill.

Almost every right that workers have today has been won by unions, sometimes violently. 40 hour work weeks, weekends off, vacation days, sick days, health benefits, minimum wage, overtime pay, and many others are all from the collective bargaining power of workers unions.