A referendum is when a government asks the general public to make a decision on an important issue.
Voting in a referendum is a bit like voting in an election. People visit their nearest polling station and respond to a question with a Yes or No answer by putting an X in a box next to the option they prefer.
The government can decide to call a referendum on any issue. But in the UK referendums tend to be quite rare and are reserved for major issues.
For example, the last referendum to be held in the UK was in September 2014 when the people of Scotland were asked the question: Should Scotland be an independent country?