Okay kiddo, have you heard of the Holy See? It's like the headquarters of the Catholic Church and it's based in Vatican City, which is a small country in the middle of Rome (that's the capital city of Italy).
Now, the foreign relations of the Holy See refers to the way that the leaders of the Catholic Church communicate and work with other countries around the world. This is important because there are Catholics all over the world and the Holy See wants to make sure that they can practice their religion and be safe and happy wherever they are.
The leaders of the Holy See, like the Pope (who is the head of the Catholic Church), have meetings and talks with leaders of other countries. They may discuss things like peace, human rights, and aid to countries in need. They also have ambassadors (kind of like representatives) who live in different countries and help to promote good relations between those countries and the Holy See.
It's like making new friends and keeping old ones, but on a really big scale! The Holy See wants to be friends with everyone so that they can spread their message of peace and love all over the world.