What does aortic overriding mean with tetralogy of fallot?

Over a VSD. The aorta is normally attached to the left ventricle and carries oxygenated blood to the body. In tetralogy of fallot, the aorta arises over a ventricular septal defect (vsd, a hole between the left and right ventricles of the heart). This means that unoxygenated blood from the right ventricle also flows into the aorta.