What is GDP and how is it measured?

As we’ve been reporting, the UK’s gross domestic product (GDP) fell by 20.4% in April, the largest fall since monthly records began in 1997, as the full impact of the coronavirus lockdown was felt.
In the three months to April, GDP fell by 10.4% compared with the previous three-month period, according to the Office for National Statistics.
But what exactly is GDP, and how is it measured in the UK?