Okay, imagine you're playing with your friends and you want to be the leader of the game. You could be the regular leader, but what if you were in charge of ALL the games you played? That would make you the High King of All Games!
In real life, a High King is like the boss of an entire country. They have the highest level of power over all the other leaders and people in the country. It's like if you were the High King of your school, you would have the final say in everything that happens there.
High Kings used to be common in places like Ireland and Scotland a long time ago. They would be in charge of different kingdoms or tribes, but they would be the ultimate ruler of them all. It's like being the biggest fish in the pond!