US GDP comes in three estimates (advance, second, third) for each quarter. Two consecutive negative quarters is the popular definition of recession, though the official call is made separately.
GDP is backward-looking and heavily revised, so it moves markets less than cpi or nfp. It matters most when it surprises sharply.
Example: advance Q1 GDP prints +1.6% annualized against a 2.5% consensus, with an inflation component running hot. Stocks and bonds both fall.
Related: unemployment-rate, cpi, pmi, economic-calendar