GDP is the primary indicator of economic size. Only final goods and services are counted: intermediate products are excluded to avoid double counting. The indicator is calculated quarterly and annually.
There are nominal and real GDP. Nominal GDP is measured in current prices and grows partly due to inflation. Real GDP is recalculated in base year prices and shows changes in physical production volume — this is what is used when discussing economic growth rates. GDP per capita is calculated separately, allowing comparison between countries of different sizes.
For individual investors, GDP is a background reference point rather than a working tool. Data is released with a lag and subject to revision, and the link between economic growth and stock market returns is indirect: the market reflects profit expectations for specific companies, not the overall volume of production in the country.