Okay kiddo, so you know how sometimes there are bad chemicals that can hurt people, animals, or the environment? Well, scientists and companies are working hard to find other chemicals that can do the same job but won't hurt anyone or anything.
It's kind of like when you want ice cream, but your mom gives you frozen yogurt instead. They're both cold and sweet, but one is healthier for you.
Using these "safer" chemicals instead of the dangerous ones is called "substitution." It's like choosing the better option so we can keep everyone and everything safe and happy.