Unilateralism is like when you want to do something all by yourself without asking other people if they want to help or what they think. So, if you want to play with your toy all by yourself without sharing it with your friends, that is being unilateral.
Now, the United States is a big country with a lot of power. Sometimes, the leaders of the country want to make decisions all by themselves without talking to other countries or asking what they think. They might do this because they think it is the best way to do things, or because they think that other countries might not agree with them. This is what is called unilateralism in the United States.
Just like how sometimes you might want to play with your toy all by yourself, leaders in the United States might want to make decisions all by themselves without asking other countries what they think. But sometimes, it's important to talk to your friends and ask what they think, just like how it's important for leaders to talk to other countries and work together. It's all about finding a balance!