When you're 5 years old, it's hard to understand what laws are and why they exist. But let's try to explain it anyways.
A long time ago, a lot of people used a plant called cannabis to feel better. They would smoke it or eat it, and it would make them happy, sleepy or help them with pain. But some people thought it was bad and they made it illegal.
In the 1930s, the government of the United States made a law that said cannabis was really bad and nobody should use it. They said it was dangerous and could make people sick. People who used cannabis were put in jail or fined a lot of money.
But over time, people started to see that cannabis wasn't actually as bad as they thought. Some people even said it helped them feel better when they were sick. So some states decided to make their own laws that said it was okay to use cannabis if you had a medical reason for it.
More recently, some states have also made it legal for adults to use cannabis recreationally. This means that you can use it just for fun, like having a glass of wine with dinner.
But even though some states have made it legal, it's still against federal law in the United States. That means that the government can still arrest people who use it, even if it's legal in their state.
So, the timeline of cannabis law is a long and complicated one. But it basically started with people using it for many years, then it was made illegal, and now some places are making it legal again.