This is the modern day version of the Domesday Book. UK GDP is the total value, in pounds and pence, of all goods and services produced over the course of a year in the UK and is currently around £2 trillion per year. It is measured by the Office for National Statistics and published four times a year, although this is split up into a preliminary, second and final estimate as more data becomes available. People usually talk about percent changes in GDP rather than the amount in pounds because it is growth that matters. Inflation is usually subtracted from GDP i.e. people talk about real GDP. In some cases inflation is not subtracted in which case this is nominal GDP.
If growth turns negative for several quarters in succession this is called a recession and is bad news for companies, whose earnings typically fall as economic activity slows down, and workers whose jobs become more precarious as companies lay off staff to cut costs. The purpose of measuring an economy’s GDP is primarily so the government can see how much tax it can raise. In fact the disciplines of statistics and economics owe their existence in large part to measurement of Gross Domestic Product. For investors GDP is not as useful as Purchasing Manager Indices because it is backward looking. Equity markets usually sell off far in advance of a formal recession.