Essentially, it means that the brain is not getting enough oxygen. Oxygen is carried to the brain in the blood vessels, and anything that deprives the brain of oxygen leads to a situation where the nerve cells in the brain die. Let me give you an example. If someone has a significant blockage in an artery going to the brain, the brain does not get enough oxygen and patients may have symptoms of a stroke. When these symptoms, like loss of vision, numbness, weakness, dizziness, vertigo, severe headache, when these symptoms occur, very briefly, and then resolve, they are considered 'transient' and are called 'transient ischemic attacks'.
Ischemia is a medical term for lack of oxygen, essentially.
When the brain is deprived of blood flow for too long, and the cells go on to die, that is considered a stroke. There are two broad categories of stroke. The first we've been talking about is ischemic, where the brain does not get enough blood flow. And, what that means, in essence is that there has been bleeding into the brain. So, neurosurgeons are trained to identify or recognize these symptoms of a stroke in a patient to decide whether the stroke is ischemic or hemorrhagic, and then very quickly determine what is the best treatment for a patient having a stroke.