Okay kiddo, so you know how some people look different from others, like some have light skin and some have dark skin, right? And they have different traditions, cultures, and languages too.
Well, in the United Kingdom, they want to know how many people of different ethnic groups live there. So they ask people to fill out a form and choose which ethnic group they belong to.
The options are things like "White British" for people who were born in the UK with British ancestors, "Black African" for people who were born in Africa with black skin, or "Asian Indian" for people who were born in India with brown skin.
These groups are then counted up together, so the government can see how many people of each ethnic group live in the UK. This helps them make sure that everyone gets equal rights and opportunities, no matter where they come from or what they look like.