Gravity drives are a form of propulsion that uses manipulations of space-time to move a ship. This is considered a 'reactionless' form of propulsion as it does not require any reaction mass to be propelled (unlike ion drives).
Gravity drives work exactly like subspace drives, except instead of creating a subspace bubble, the drive simply creates a gravity well in the desired direction of movement. This allows the ship to "fall" towards the desired location.
Most ships use their subspace drives as gravity drives, just at a lower power. This is very convienent, but often inefficient. Unless the ship's drives have been designed to be used in this way, they can be very inefficient at lower power levels.
One of the problems with gravity drives is that while they can, in theory, accellerate a ship near instantly to a good fraction of the speed of light, the gravitational sheer put on the ship is often enough to tear it appart. Because of that, gravity drives often have to serve the dual purpose of accelerating the ship, while also combating their own gravitational sheer to keep the ship from being crushed. This puts a rather steep limit on a ship's ability to accellerate, relative to the mass of the ship. Smaller ships can accellerate faster, in general, but have less power available, while larger ships can have larger reactors, but their mass exponentially increases the energy required to accellerate them.