It stands for Gross Domestic Product. It is supposed to measure the strength of the economy.
There are several ways of measuring it.
You could measure all the money in the economy, and how many times the money circulated in a year.
Or you could measure all the sales for the year.
Means nothing when not using market prices, the government is 40-50% of GDP. So GDP is meaningless.
If the government provides you a service at a cost that you would have not paid yourself, and values that cost in the GDP figures then it is not a true reflection.