Ad gentes is a Latin phrase used to talk about spreading the message of Christianity to people who haven't heard it before.
Imagine you have a really yummy candy and you want to share it with all your friends. You tell your friends who are right next to you, but there are other friends who live far away and don't know about your delicious candy. So, you need to find a way to reach them and let them know about your candy.
In the same way, Christians want to share the message of Jesus, who they believe is really important, with everyone in the world. But there are some people who live in places far away, where no one has told them about Jesus. So, Christians need to go to those places, learn about the people, and tell them about Jesus in a way that they can understand.
This is called ad gentes, and it means going to the "nations," or people who haven't heard about Jesus yet, and sharing the gospel with them. This is an important part of Christianity because Christians believe that everyone should have the chance to hear about Jesus and decide if they want to follow him or not.