Okay kiddo, so a long time ago in history, there were some very important people in the Catholic Church called Cardinals. They were like the bosses of the church and helped make important decisions.
Now, sometimes a Cardinal would have a nephew (that's like a son, but it's the Cardinal's brother or sister's child) who he loved very much. This nephew would be given a special job in the church, kind of like being an assistant to the Cardinal.
This job was called being a Cardinal-Nephew. The Cardinal-Nephew would help the Cardinal with important church matters and sometimes even be in charge of the church while the Cardinal was away or busy doing other things.
However, sometimes people thought that being a Cardinal-Nephew was a bit unfair because they were given important positions just because they were related to the Cardinal, even if they were not as qualified or deserving as other people.
So while being a Cardinal-Nephew was a big deal and important job, it wasn't always seen as fair to other people who wanted to work in the church too.