Okay kiddo, so imagine that you and your friends have a secret that you promised each other you won't tell anyone else. That secret is kind of like money that you keep safe and don't give to others.
Now, there are some people who belong to a group that also has secrets, and they promise not to tell those secrets to anyone who is not part of their group. This is called Mesirah.
For example, if someone who belongs to this group finds out about your secret, they won't go running to tell their friends or family about it because they promised to keep their group's secrets safe. It's like a game where everyone agrees to play by a certain set of rules.
However, sometimes people use this Mesirah rule to protect other bad people who have done something wrong. When this happens, it's not okay anymore because it's important to tell the truth and help protect others from harm.
So, Mesirah is like a secret promise between people who belong to a certain group, but it should never be used to hide bad things that happen.