Imagine you and your friends all have different toys, but you still want to play together. You might need someone to help organize the games and make sure everyone is playing fair. The Secretary of State for Commonwealth Affairs is kind of like that person, but for a group of countries called the Commonwealth.
The Commonwealth is a group of 54 countries that have some things in common, like a shared history with Britain and a commitment to certain values like democracy and human rights. The Secretary of State for Commonwealth Affairs is a person who works for the government of the United Kingdom and helps to manage relationships with these countries.
One of the Secretary's jobs is to communicate with other Commonwealth countries and represent the United Kingdom at international meetings, like the Commonwealth Heads of Government Meeting. They also work with other government officials and departments to make sure policies and decisions take into account the interests of other Commonwealth countries.
In short, the Secretary of State for Commonwealth Affairs is like a grown-up toy organizer for a group of countries that like to play nice with each other.