Something that's subdural is located under the outermost membrane that protects the brain. After a nasty tackle, a football player might be diagnosed with a subdural hematoma, or bleeding on the brain.
You'll almost always come across the adjective subdural in a medical context — either at the doctor's office or in a an anatomy textbook, for example. It's used for something beneath the outer layer of membranes around the brain, which are known as the dura or dura mater. Add the Latin prefix sub- ("under") to dura and you get subdural, or "under the dura."