It's a wanky term these days, due to overuse and marketing.
Which is a shame, because it's most of the wine.
Terroir (pronounced tehwa) is a French word for pretty much everything in the vineyard from soil to sun to water and air. It's the climate and everything that might influence anything.
When the point of winemaking is to show the vineyard and vintage, terroir becomes everything. It's what creates the starting product, and everything you do is aimed at preserving that, and not dulling or perverting it's intricacies.