A Grav Drive is a term which actually refers to three distinct concepts working together to control gravitic attraction and inertia, allowing for the ease of space travel in The Verse.
The first of these advances is known as screening, which reduces the pull of gravity experienced by two objects relative to each other. While screening doesn't change an object's inertia, it does place it in an essentially zero gravity environment; for example a spacecraft no longer has to fight the planet's gravity to remain airborne and so can save its fuel for getting up to orbital velocity. While it takes a lot of time and energy to get a g-screening field working, once it is going it takes very little energy to remain operational.
The second is artificial gravity, which produces the opposite effect of a screening field to pull objects towards it. Artificial gravity is used to simulate the effects of normal gravity on a spaceship operating a screening field, and while it cannot change the forces of ordinary acceleration careful management can negate some of the more violent effects. This same g-field technology was used in terraforming projects on the smaller worlds to allow for the existence of a breathable atmosphere.
The final is inertia reduction, which is simply a matter of a spaceship aligning its g-field and g-screen against each other in a particular manner, dropping the ship's resistance to motion in a given direction to almost zero. This trick is particularly important in the function of a pulse drive, which is what allows most boats to cross interplanetary distances in such short times.