Okay kiddo, so you know how we have a bunch of different churches like the one we go to on Sundays or the one grandma goes to with the big cross on top? Well, there's something called the Catholic Church, which is the biggest one of them all.
Now, the Catholic Church believes that Mary, Jesus' mom, is super special. They think that she's not just a regular mom, but the "mother of the Church" too.
So, just like how you have a mom who takes care of you and loves you, the Catholic Church believes that Mary takes care of all the people who go to church and loves them too. They think that since Mary said yes to being Jesus' mom, she's kind of like the mom of all the Christians too.
It's kind of like a big family, with God as the dad, Jesus as the big brother, and Mary as the mom who looks after everyone. And since the Catholic Church is the biggest family of Christians, they call Mary the mother of the Church.
Does that make sense?