Imagine you have a special toy that nobody else in your neighborhood has seen before. When your friends come over to play, they are amazed by it and want to know where you got it from. You become like a 'light' that shines on the toy and makes it known to everyone.
Now, think about a grown-up who knows something really important and valuable, which can help others. They can become like a 'light' that shines and shares their knowledge with people around the world. This is what "light unto the nations" means - doing good things and sharing positive knowledge to help others.
So, just like when you bring your special toy to share with your friends and become a 'light,' adults can bring their knowledge and skills to help make the world a better place, and they become a 'light unto the nations.'