Getting a concussion is bad. But getting more than one can be significantly worse. Doctors use the term second impact syndrome to describe the aftereffects of multiple concussions in a short period of time. It can be deadly. 
Concussion happens when the brain moves beyond its normal limits. With a blow or jolt to the head or body, the head and brain move back and forth rapidly. These sudden movements can make the brain bounce or twist inside the skull.  It’s also known as mild traumatic brain injury. The new name came from recent efforts to make it sound as serious as it actually is.