Imagine you are on an airplane and all your friends are sitting with you. Now, let's say that you want to get off the plane and go to a different place, but your friends want to stay on the plane and go to the same place as before.
This is kind of like what happens with a jet group. Jets are airplanes that fly really high up in the sky, and sometimes they fly in groups together. When a group of jets is flying together, they call it a jet group.
Just like with your friends on the plane, sometimes one jet in the group might want to go to a different place or fly at a different speed than the other jets in the group. When this happens, that jet might leave the group and go fly by itself.
But usually, the jets in the jet group want to stay together and fly at the same speed and go to the same place. It's kind of like how you and your friends might stick together and go to the same place in the airport after you get off the plane.
So, a jet group is just a bunch of airplanes (or jets) that are flying together and staying close to each other while they fly.