"Mini-stroke" is a term that commonly refers to the medical condition Transient Ischemic Attack (TIA). TIA refers to loss of neurologic function that is short-term (less than 24 hours). These occur due to blockages or abnormalities within the blood vessels of the brain. TIAs are warning signs for a stroke. A stroke results in permanent damage to the brain due to the same vascular issues restricting blood flow, but lasting a longer time.
If someone has a TIA they should be evaluated urgently, usually in the Emergency Room, or by a neurologist.
Common symptoms of TIA include sudden onset of weakness, numbness, vision loss, slurred speech or difficulty with speech. If you experience of any these symptoms you should seek immediate medical attention (within minutes -- do not wait for the symptoms to improve).