Okay, so you know when important people visit your school or your house, there is always someone who welcomes them and takes care of them? That person is kind of like the leader or the boss of the visit.
Now, imagine that the important person is from another country and they are visiting your country. The person who takes care of them is called the head of mission. This person is in charge of making sure that the visitor is safe, comfortable and that they have everything they need.
The head of mission also helps to build good relationships between the two countries, just like how friends help each other and make each other happy. They do this by talking to the visitor and learning about their culture and traditions, and making sure that everyone respects each other.
So, the head of mission is like a big boss who takes care of important visitors and makes sure everyone is happy and safe.