Alright kiddo, so a long time ago, there was a man called Pope Pius XII who was in charge of the Catholic Church. He decided to make some changes to the way people do things during Mass, which is a special meeting for Catholics to pray and worship together.
One thing he did was update the words that priests say during Mass. He wanted the words to be in a language that more people could understand, instead of just Latin. He also made the words simpler and easier to remember.
Pope Pius XII also made changes to the music that is played during Mass. He wanted the music to be more peaceful and calming, so that people could focus on their prayers and worship. He also allowed more different types of music to be used, instead of just one style.
Another change that Pope Pius XII made was to the way priests dress during Mass. He decided that priests should wear simpler and more modest clothing. This way, they would not be distracting from the worship service.
Overall, Pope Pius XII made these changes to make the Catholic Church more understandable, calming, and meaningful for people who attend Mass.