ViewVertical is an option for Graphics3D and related functions which specifies what direction in scaled coordinates should be vertical in the final image.
The default setting is ViewVertical->{0, 0, 1}, which specifies that the z axis in your original coordinate system should end up vertical in the final image.
The setting for ViewVertical is given in scaled coordinates, which run from 0 to 1 across each dimension of the bounding box.
Only the direction of the vector specified by ViewVertical is important; its magnitude is irrelevant.
Changing ViewVertical has the effect of tipping a simulated camera.