Imagine you have a pet fish in a bowl. One day, you forget to feed the fish and it dies. You don't know how long the fish has been dead because you didn't check on it for a few days. The time between when the fish died and when you found it is called the post-mortem interval.
Post-mortem interval is a big, fancy word that scientists and doctors use to talk about how long a person or animal has been dead. They use different clues to figure this out, like how stiff the body is, what kind of bugs are around it, and how much the body has started to decompose. This helps them figure out important things about what caused the person or animal to die and if there were any other factors that contributed to the death.
So, to sum it up, post-mortem interval is basically just the amount of time that has passed since someone or something died. It's important because it helps scientists and doctors understand more about the circumstances surrounding the death.