Okay kiddo, imagine you're playing in the playground with your friends. You're all having a great time playing on the swings and slides.
Now let's say there is a really big kid who is playing too rough and is hurting other kids. The adults who are watching over the playground have a duty to warn you and your friends about the big kid so you can stay safe and avoid getting hurt.
This is called a "duty to warn." It means that when someone knows about a danger or threat that could harm others, they have a responsibility to tell those who could be harmed so they can protect themselves.
Sometimes this duty to warn applies to grown-ups too. For example, if a doctor knows that a patient is going to hurt someone else, they have a duty to warn that person or the authorities to prevent harm from happening.
So just like the adults watching over the playground have to warn you when there's a dangerous kid, some people have a duty to warn others when there's a threat that could cause harm.