Literally '(classified) great growth'. The meaning varies by region.
Within Bordeaux, grand cru class refers to the top estates of Medoc, Graves and Sauternes, and the top wines of St Emilion. In Burgundy and Chablis the very top vineyard sites are referred to as grands crus (which is a higher classification than premier cru). Alsace also has vineyards classified as grands crus (fairly recently), and in Champagne this designation is used for the most highly rated vineyard sites (and by extension the wines made exclusively from them).