What exactly is chronic subdural hematoma?

Blood around brain: There are three layers of tissue that surround the brain, known collectively as the "meninges." the outermost layer is the dura mater, the middle layer the arachnoid mater, and the layer closest to the brain the pia mater. When veins that go between these layers rupture - often because of trauma - blood can start to pool between the dura mater and arachnoid mater causing a subdural hematoma.

Subdural hematoma: A chronic subdural hematoma typical occurs after a head injury but does not present with symptoms until days to weeks after the event. It is a collection of fluid that develops between a membrane over the brain called the dura and the brain itself. If symptomatic these often need to be drained.