The longer answer
The stock is the half of the shotgun your body actually touches. The shoulder finds the butt at the back, the cheek finds the comb on top, and the trigger hand wraps the grip in the middle — three contact points that, together, hold the gun's sighting plane in line with your eye. Everything we mean by "gun fit" lives in the stock: comb height sets where your eye sits relative to the rib, and length of pull sets how far the trigger is from your shoulder.
Walnut is the traditional material and still standard on nicer field guns. Clay-sport shooters often prefer laminated wood or synthetic stocks because they hold their dimensions across humidity and temperature swings — a stock that swells half a millimetre in August is a stock that shoots somewhere different than it did in March.