Free floating means there is no pressure on the barrel from the receiver all the way up to end of the forearm. Most factory guns have a pressure point here and there do the stock doesn't fit the barrel perfectly. These pressure points can cause accuracy problems possibly due to vibrations not being consistent in the barrels as bullets fly out or cause stringing as the barrel heats up.

Glass bedding usually only takes place on the recoil lug and all around the reciever. This is just another way that the you have a nice tight fit for the gun to stock. Its just a fiber glass like mixture that you apply in the stock. Alot of people also go another step and pillar bed with aluminum pillars the the screws that hold the reciever to the stock. When the gun is fired, this will ensure that reciever doesn't move in the stock as the bullet is going down the barrel. Also this nice fit will cut down on the vibration going on on the barrel so it is more tolerant to different bullet depths.

Some people don't like a free floated barrel. They like a little pressure on the barrel at the end of the stock. I believe this method works good if you have a good strong stock like a McMillian or even H-S precision. But if you got a cheap plastic stock like remington puts out on their supposely synthetics, I believe its better to free float.