Vertebrobasilar insufficiency is a medical condition characterized by reduced blood flow to the posterior part of the brain, which includes the brainstem, cerebellum, and occipital lobes. This reduction in blood flow is typically caused by the narrowing or blockage of two vertebral arteries by the buildup of plaque, leading to transient ischemic attacks (TIAs), "mini strokes" that result in the temporary loss of brain function.