Well, sweetie, an interrex is like a special person who's in charge when the normal leader of a country is not there. It's kind of like when your teacher is sick and a substitute teacher comes to your class instead.
In ancient Rome, when a king or an important leader died, the interrex would be chosen by a group of people to rule for a short period of time until a new leader was elected. This was important because it helped to keep things going smoothly during the transition between leaders.
So, think of an interrex as a temporary leader who keeps things under control until a permanent leader can be chosen. It's kind of like a back-up plan so that everything can keep running smoothly.