Exactly what gorse said.

Of course there are different types of bedding.

Glass bedding gives you 100% contact around the action or recoil lug so that nothing moves around on you from shot to shot. Basically it makes a form fitting shell for the rifle to sit in. It also strengthens the stock some what and keeps places like the recoil lug area from spliting.

Pillar bedding keeps the stock from compressing over time. This allows a person to get consistant torque on the action screws with out compressing the wood. When you take a rifle out of its stock and then put it back in and tighten the screws back you willusually turn them a little bit more each time and compress the stock. Also if you do not get the screws to the same torque each time your point of impact could change on you.

A bedding block is an aluminum "block" that is skeletonized and bedded into the stock. It is precicely machined to fit the barreled action and they mate up perfectly. This accomplishes both of the above but also adds extreme rigidity to the stock itself.