Counterfactualism is imagining what would happen if things were different than they really are. It's like playing make-believe with your toys, except instead of pretending that your toy car is flying or your doll is talking, you're pretending that things in real life happened differently.
Let's say that you spilled your juice on the table. If you start thinking about what would have happened if you hadn't spilled it, that's counterfactualism. You might imagine that everything stayed clean and that you didn't have to clean it up.
Counterfactualism can be useful in helping us make decisions, because it allows us to consider different outcomes and choose the best one. For example, if you're trying to decide whether to go to a party or stay home, you could think about what would happen if you went and what would happen if you stayed. You could imagine having fun with your friends or feeling tired the next day, and use that to decide what to do.
Overall, counterfactualism is just a fancy way of talking about imagining different possibilities. And when you're young, imagination is one of the most important tools you have!