Well kiddo, a long time ago, people didn't know much about how medicines and other products would affect animals and people. So they started using animals to test these things.
At first, they used animals like rats and mice because they were easy to find and breed. But eventually, they started using other animals like dogs, rabbits, and monkeys too.
Some people didn't like this because they thought it was cruel to the animals. They thought that animals should be treated better and not be hurt for our benefit.
Over the years, laws and regulations were put in place to try to make sure that animals were treated better during testing. Scientists also started to look for other ways to test products that didn't involve animals, like using computer models.
Today, animal testing is still used but it's much more regulated and there are rules that people have to follow to make sure that animals are treated well. People are still trying to find better ways to test products without using animals so that no animals have to be hurt for our benefit.