"Mirae Caritatis" is a big fancy phrase that means "amazing love" in Latin. It was used to describe a document that was written a long time ago by the Pope, who is kind of like the boss of the Catholic Church.
The document talks about something called the "Blessed Sacrament," which is when people in the Catholic Church believe that bread and wine actually become the body and blood of Jesus Christ during a special ceremony called the Eucharist.
The Pope wanted to remind people about how important this ceremony was, and how much God loved them. He said that when people take part in the Eucharist, they are becoming closer to God and are receiving the ultimate gift of love from God.
So "Mirae Caritatis" is basically like a big love letter from the Pope to the people in the Catholic Church, reminding them how much God loves them and how special the Eucharist is.