So imagine you have a bunch of toys, and you and your friend want to play together. But sometimes you don't agree on which toy to play with or how to play with it. That's kind of like how Wales felt about being part of the United Kingdom (which is like a big group of countries that work together).
Unionism is when people in Wales think it's better for them to stay part of the United Kingdom. They believe that they're stronger and safer when they work together with the other countries in the United Kingdom, like England, Scotland, and Northern Ireland.
But some people in Wales don't agree. They think that Wales should be its own country, separate from the United Kingdom. They believe that Wales would be better off if it made its own decisions and didn't have to rely on other countries.
Just like with your toys, there are different opinions about what's best for Wales. Some people like to play alone, while others prefer to play with friends. It's up to the people of Wales to decide which they think is the best option for them.