The Bathmate is a male enhancement device classified as a hydro pump, which means it uses a combination of water and vacuum pressure to increase penis size. It does this through the use of a hollow cylinder in which the male member is placed with water filling the empty space instead of air.
Once the cylinder is filled with water and your manhood inserted, you press the base of the cylinder firmly against your pelvis. Then, every time you pull the cylinder towards you, some of the water inside is ejected from the valve situated at the tip. Doing this decreases the internal pressure which in turn causes the tissues of your penis to expand.
The fact that water is used instead of air makes it more comfortable and at the same time eliminates the safety concerns typically associated with traditional air based pumps. If used according to instructions while giving yourself sufficient time to adapt to more aggressive levels of training, it is completely safe.