Okay, so the Holy See is like the leader of the Catholic Church. They live in a special place called Vatican City, which is its own country. The United Nations is a big group of countries that work together to try and make the world a better place.
Sometimes the Holy See wants to share their opinions on important issues with the United Nations, like human rights or peace. So, they send a representative to talk to the leaders of the United Nations.
It's sort of like when you have a really good idea and you want to tell your teacher about it. You have to go to their classroom and talk to them to share your idea. The Holy See does the same thing with the United Nations.
But, the important thing to remember is that the Holy See is not a regular member of the United Nations like other countries. They have what's called "observer status," which means they can watch what's going on and talk to the leaders, but they can't vote or make decisions like the other countries can.
So, the Holy See and the United Nations are two separate things, but they work together to try and make the world a better place.