Brexit is when the UK (United Kingdom) decided to leave the European Union (EU). This means that the UK decided to no longer be part of the group of countries in the EU. In order to leave, the UK had to reach an agreement with the EU about things like what would happen to people living in the UK and the EU, how people and goods would move between the UK and the EU, and how much money the UK would need to pay the EU to leave. This agreement is known as the 'Brexit withdrawal agreement'.