Interregnum is a big fancy word that means there is no king or queen on the throne in a country. It's like when mommy and daddy go out for the night and grandpa comes to stay with us, only grandpa is not a king.
When a king or queen dies or steps down from the throne, a new one needs to take their place. But sometimes, it takes time to figure out who that new king or queen should be. During that time, there is no one ruling the country, and that's what we call an interregnum. It's kind of like when we play musical chairs, and when the music stops, everyone rushes to grab a chair, but there are not enough chairs for everyone, so some people end up standing until a new game starts.
So, during an interregnum, the country might not have a leader in charge who can make decisions and take care of things like the economy, the army, and the laws. That's why interregnums can be a little tricky, and people might get worried or scared. But don't worry, eventually, a new king or queen will step up, and things will go back to how they were before.