Okay kiddo, when people talk about a "crime against nature," they are usually talking about something that harms the environment or animals in it. You know how we need to take care of our bodies by eating healthy food and exercising? Well, the environment and animals also need to be taken care of so they can stay healthy and alive.
Sometimes, people do things that hurt the environment, like throwing trash on the ground or cutting down lots of trees. These are examples of a "crime against nature" because they harm the natural world around us.
Also, sometimes people do things that hurt animals, like hurting them on purpose or taking their homes away. This is also a "crime against nature" because animals are part of our environment and hurting them can hurt the whole ecosystem.
So basically, a "crime against nature" is when someone does something bad that harms the environment or animals in it. We need to protect our planet and the creatures that live on it, so we need to make sure people don't do these kinds of bad things.