Ok, so you know how sometimes your family might have a king or a queen who makes rules for everyone to follow? The United Kingdom is kind of like a big family, and they have a queen named Elizabeth II who is the head of their government.
Now, unlike most families where the king or queen is chosen by birth, the United Kingdom has rules about who can become the king or queen. This is called the royal succession. The first person in the line of succession is usually the oldest son or daughter of the current king or queen, but sometimes other family members can be next in line too.
The monarch (that's just another word for king or queen) mostly does things like representing the country at important events, meeting with important people from other countries, and signing laws that the government makes. They also have a lot of ceremonial duties, like opening new buildings and giving speeches.
But the monarch doesn't actually make the laws or run the government - that's done by a group of people called parliament. The UK has a parliamentary democracy, which means that people get to vote for who they want to represent them in parliament, and those people make decisions about how the country is run.
So even though the queen is the head of the government, she doesn't really have that much power. Her role is mostly to be a figurehead who represents the country and its traditions.