Tribal knowledge

Tribal knowledge means that there are things that people in a group know, that aren't written down anywhere. It's like a secret code of information that only they know. Imagine that you have a special game that only you and your friends know how to play. You might not have written down the rules anywhere, but you still know how to play it because you all taught each other. That's kind of like tribal knowledge. It's kept within a group and passed down from person to person, so that everyone in the group knows it.