In my own words: A labour union is a group of workers who have banded together for better wages, benefits, and conditions at their workplace.
If you like those things, you should organize a union with your coworkers. If you already have those things, you should organize a union to make sure you keep them.
... And if only it was that easy to "just organize". Here's why I think it's hard to form a union:
Most people support unions. But some people, especially younger workers, don't even know what unions are. (I didn't know what they were until my mid-20s...) If there are less people in unions, then less people know someone in a union, let alone what unions actually do for workers.
Another issue is time and effort. Employers are good at squeezing more hours and energy out of workers than necessary. To maximize their profits, they minimize employee wages. They also minimize downtime: breaks, rest, even meals. Employers hate unions because it means they would have less control over these things. In turn, that means less control over their profits - how much they make off of the work their workers produce. The work you do.
Employers usually earn a lot more than their workers because that's how they and people like them designed the system. They use their excessive wealth, time, and control to prevent unions - union busting. Examples of union busting might include intimidation, or creating divisions between you and your coworkers.
A coworker might mysteriously get a raise or benefits, while others get nothing. An entire staff might get punished for something one worker did. A coworker is reprimanded for speaking another language during their break.
It's really damn hard to organize. Workers that need a union the most are often ones with low wages. That usually means they don't earn enough for savings. There's nothing cheaper in the store - they just go without. Low wage jobs also usually mean low job security. Even if workers win a union vote, employers can stall the first contract and make their workers' lives awful in the meantime. Not all workers can afford a fight, because the system is rigged against us.
But no matter how much money employers spend trying to sabotage union efforts, workers still win. Workers still unionize. They all started somewhere, and that's organizing. Talk to your coworkers. Share your grievances with each other. Think about what you'd like improved at your workplace. The 8-hour workday was once 10, and even longer before that. It took time and effort from workers to fight for better. Things can change, and they should.
The workplace is where most of us are. Us being people who have to work for a living. We give the best hours of the best days of the best years of our lives to employers, as economist Richard Wolff has said. It's worth improving your workplace, for yourself and your coworkers.