Okay little one, so the Supreme Governor of the Church of England is like the boss of the church. This person gets to make important decisions and has a lot of power over how the church is run.
Now, the thing is, in England, the queen or king is actually the official Supreme Governor of the Church of England. This means that they are in charge and have the final say on things like choosing bishops and making changes to the church's official teachings.
But, since the monarch is usually very busy running the country, they appoint someone else to act as the Supreme Governor on their behalf. This person is usually the Archbishop of Canterbury, who is like a really important leader in the church.
So, to sum it up, the Supreme Governor of the Church of England is the person who makes the big decisions and has lots of power over the church, and in England, it's usually the Archbishop of Canterbury acting on behalf of the monarch.