Order of succession is a fancy way of saying who gets to be the leader when the current one stops being in charge.
Think of it like playing a game with your friends. If you’re playing a game where you have to take turns being the leader, you need to know who gets to be the leader next when your turn is over. That's what order of succession means.
The order of succession is usually written down in a list, starting with the person who is currently in charge (like the president of a country, or the CEO of a company).
If anything happens to the leader (like they die, get sick or quit), the next person on the list becomes the new leader.
This list usually goes all the way down to the last person who could possibly take over. It’s important to have an order of succession so that everyone knows what to do in case something unexpected happens.