Okay kiddo, let me explain it to you in a way that's easy to understand.
When people talk about being called to a common mission, they mean that they share a goal with other people and they are working towards it together. Just like you and your friends might work together on a project for school, or you and your family might work together to clean the house.
Some people believe that they have a special purpose in life, and that this purpose is given to them by a higher power. This could be God, the universe, or something else. They feel like they are meant to do something important, something that will make the world a better place.
When a group of people feel this way, they might come together and form a team or a community with a common mission. They might work towards a goal like helping people who are sick, protecting the environment, or fighting against injustice.
Being called to a common mission can be very inspiring and motivating, because it means that you are part of something bigger than yourself. You have a sense of purpose and belonging, and you can work together with others to make a difference in the world.