A subdural hematoma is a hematoma (bloodclot) that occurs underneath dura mater. In other words, it is a bloodclot that occurs in the middle of the brain, not on the top of it. A blodclot in the dura mater itself is called a dural hematoma.
Depending on where the hematoma is located and how large it is, a hematoma can have a very large effect or hardly be noticable. But since this is a big piece of scar tissue right in your brain, there is obviously risks involved.
Other then the problem that a subdural hematoma means that their has been bleeding in the brain, and thus an interrupted blood supply, the danger of the clot comes from the fact that it is applying pressure to structures around it. This can compress nervous tissue, making it temporarily or permanently non-functioning.
If the area being compressed is in the cerebral cortext, which is very large and mostly unconcerned with the regulation of life processes, there is a good chance that the subdural hematoma can be survived. The victim, may, however, lose their ability to speak or move, as with a stroke victim. If the clot occurs in a lower area, such as the thalamus or the respitatory nuclei of the medulla oblongata, there is a good chance that even a very small clot can cause immediate death.
Another way that this kind of hematoma can be dangerous is if it blocks a vein that drains the cerebral cavity of CSF. If this happens, CSF will build up in the brain until it is at such a great pressure that it can compress brain tissue.
The usual cause of a subdural hematoma is either spontaneous bleeding in an artery wall due to old age or congenital defect, or else a sharp blow to the head.
Treatment can be either to treat the symptoms until the issue resolves itself, or if the clot is life threatening enough, surgery to remove it.