Okay kiddo, have you ever heard of a very important person called the Pope? Well, when the Pope passes away or resigns, there is a time when no Pope has been chosen yet. During this time, it is called sede vacante which means "the seat is vacant" in Latin.
Now, you might be wondering what happens when there is no Pope? Well, there are a lot of important things that need to be done like organizing the funeral of the previous Pope, getting all the cardinals together, and deciding who the new Pope will be.
During this time, there are a lot of traditions and rituals that are followed. For example, the bells in the Vatican will toll every day to remind people of the vacancy, and the papal apartments, where the Pope used to live, are sealed.
So, in summary, sede vacante is a special time that happens when the Pope passes away or resigns, and it is a time when everyone is working hard to choose the next Pope.