What is GDP?

Gross domestic product, or GDP, is an economic term referring to the sum total monetary value of all goods produced in a particular country, as well as the value of any services performed there. Essentially, this is a way of measuring economic activity within a period of time to track growth or decline. The value of the gross domestic product is not only dependent upon the consumption of finished goods, but also investment, the value of exported goods, and government spending. 
The value of a gross domestic product is often used in reference to a nation's development and growth, as well as the quality of life for people in that nation.
Some people disapprove of the gross domestic product as an indicator of economic activity because it does not account for any unofficial, or "black market," goods and services produced by a nation.
https://www.britannica.com/topic/gross-domestic-product

https://www.investopedia.com/terms/g/gdp.asp