A groupset is a collection of bike components designed to work together. These days it usually means the gears and brakes on your bike. The term once included the hubs and headset too, but threadless headsets knocked the traditional groupset makers — Shimano and Campagnolo — out of the market.
Nevertheless, the groupset is where a lot of the money in a new bike goes. And with the introduction of electronic groupsets a few years ago and more recently hydraulic disc brake variants, the range of options just get bigger and bigger!