Okay, kiddo! So, sometimes there are big problems in countries about who should be the ruler or leader. When people want someone to be the leader, they need to find a way to make it official so everyone will agree. This is called "legitimacy."
A "legitimist" is someone who believes that the leader has to be from a certain family or group to be "legitimate." This means they think only certain people should be in charge, because they belong to a special line or group that has the right to rule.
Think of it like a game of pretend: if you and your friends are playing "royalty," you might say that only the person with the sparkly crown can be the queen or king, because that's how you decided to play the game. Similarly, legitimists believe that only certain people can be the boss of the country, because of how their rules say it should be.
However, not everyone agrees with legitimists! Some people think that anyone who is chosen or elected by the people could be a good leader, even if they are not from a certain family or group. It's up to everyone to decide what they think is fair and right, just like you and your friends deciding on the rules of your pretend game.