Okay, so have you ever played with your friends and you all have different ways of talking? Like maybe one of your friends speaks Spanish, and another friend speaks Mandarin, and you speak English?
Well, imagine if your whole country was like that! Everyone would be speaking different languages and it might be hard to understand each other. So, many countries have a "official language" that everyone is supposed to know and use when speaking to each other.
An official language is like a rule that says which language should be used for important things, like writing laws, giving speeches, or publishing books. In some countries, the official language is the same as the language that most people speak, like in the United States where English is the official language.
But in other countries, the official language might not be the same as the language that most people speak at home. For example, in Canada, English and French are both official languages because there are many people who speak French as their first language.
So, basically, official language is a special language that is supposed to be used for important things, like talking to the government or doing official business. It helps everyone communicate better and helps keep things organized.