Okay, imagine you and your friends are playing together without a teacher or leader.
In music, sometimes a group plays without a conductor, who is the person that usually helps everyone keep time. When this happens, one of the musicians will act as the leader. This person listens carefully and helps everyone else keep the same rhythm, like a game of follow the leader!
Think of it like this: if you're playing tag and one friend says, "Let’s go!" that person starts running, and everyone else follows. In music, that leader helps everyone know when to play their instruments so they all sound great together.
So, the leader helps everyone stay 'in time' or keeps the 'beat,' just like how everyone else follows along without getting lost!