Imagine you and your friends all play different games, but you all want to have a snack break at the same time. It would be much easier if you all worked together and agreed on one game to play so you could take a break together.
That's kind of like what a one big union is. It's when people working in lots of different jobs all work together as one big group to try and get better pay and working conditions. Instead of just one group of workers fighting for their own rights, they all fight together for everybody's rights.
It's like if all the kids in the school decided to join forces and ask for better playground equipment from the principal instead of just one class asking for new swings. When everyone works together, they can get more done and make sure everyone is taken care of fairly.