Tanistry is a way of picking a leader in some cultures. It's like if you and your friends were going to pick who gets to be the captain of your team. But instead of just picking the person who is the best at sports or the smartest, you pick someone who is related to the last leader.
It's kind of like passing the captain's hat down the family line. So if the last captain was someone's dad, then his son would be the next captain. Or maybe it's the uncle or cousin of the old captain who gets to be the new captain.
The idea behind this is that the new leader is already connected to the old leader, so they know what the old leader would want or how they would do things. It's like keeping things in the family, so to speak.
Some cultures still use tanistry to pick their leaders today, but others have moved away from it and pick their leaders in other ways.