An apostolic constitution is like a really important rulebook made by the leader of a big church called the Pope. Just like how your teacher makes rules for your classroom, the Pope makes rules for the church.
This rulebook isn't just any old book, it's really, really special! It's like the most important rulebook and it's called a "constitution".
It has to be written in a special way and when it's made, everyone has to follow it. The Pope writes the apostolic constitution for very important things that he wants to tell all the people in the church.
It's kind of like if your mom or dad made a really important rule for the family like "we all have to eat our vegetables at dinner every night" and everyone had to follow it.
So, to sum it up, an apostolic constitution is kind of like the most important rulebook in the world for the Catholic Church made by the Pope to tell everyone what they need to do.