A long time ago, there was a disagreement between two big groups of religious people - the Anglicans and the Catholics. They couldn't agree on who had the power to make other people priests. The Catholics said only their leaders could do it, but the Anglicans said anyone could! This dispute went on for a very long time.
Finally, in the year 1896, the Pope made a big statement called apostolicae curae. He said that the Anglicans were wrong and only the Catholic leaders had the special power to make someone a priest. This meant that any priests made by the Anglicans were not officially priests according to the Catholic Church. It caused some trouble and sadness for those Anglican priests and people who wanted to cross over to the Catholic Church.
So, in short, apostolicae curae was a statement made by the Pope that said only Catholic leaders could make people priests, and this caused some problems for Anglicans who wanted to join the Catholic Church.