Once upon a time, a long time ago, the United Kingdom (which is a country in Europe) decided to join a group of other countries in Europe called the European Communities. The European Communities were like a big club where countries worked together on things like trade and making sure everyone was treating each other fairly.
To become a member of this club, the United Kingdom had to follow a lot of rules that the other countries had already agreed on. They had to make sure they were treating their workers fairly, protecting the environment, and following the same laws about things like money and trade.
After a lot of talking and negotiating, the United Kingdom finally joined the European Communities in 1973. This was a big deal because it meant that they could work together with the other countries to make things better for everyone.
Over the years, the European Communities grew and changed, and eventually became what we now know as the European Union. Sometimes, the United Kingdom didn't like all the rules and decisions that the European Union made, and they even talked about maybe leaving the club.
In 2016, the people of the United Kingdom voted in a special election to decide if they wanted to stay in the European Union or leave. It was a very close vote, but in the end, more people wanted to leave than stay. So, the United Kingdom is now in the process of leaving the European Union and going back to making their own rules and decisions.