https://govfacts.org/federal/commerce/how-the-u-s-gdp-is-calculated/
(very good explanation and examples)
https://www.investopedia.com/terms/g/gdp.asp
Real GDP
Real GDP is an inflation-adjusted measure that reflects the number of goods and services produced by an economy in a given year, with prices held constant from year to year to separate out the impact of inflation or deflation from the trend in output over time. Since GDP is based on the monetary value of goods and services, it is subject to inflation.
Rising prices tend to increase a countrys GDP, but this does not necessarily reflect any change in the quantity or quality of goods and services produced. Thus, by looking just at an economys nominal GDP, it can be difficult to tell whether the figure has risen because of a real expansion in production or simply because prices rose.
Economists use a process that adjusts for inflation to arrive at an economys real GDP. By adjusting the output in any given year for the price levels that prevailed in a reference year, called the base year, economists can adjust for inflations impact. This way, it is possible to compare a countrys GDP from one year to another and see if there is any real growth.
Real GDP is calculated using a GDP price deflator, which is the difference in prices between the current year and the base year. For example, if prices rose by 5% since the base year, then the deflator would be 1.05. Nominal GDP is divided by this deflator, yielding real GDP. Nominal GDP is usually higher than real GDP because inflation is typically a positive number.2
Real GDP accounts for changes in market value and thus narrows the difference between output figures from year to year. If there is a large discrepancy between a nations real GDP and nominal GDP, this may be an indicator of significant inflation or deflation in its economy.