What is an aneurysm clip for, and what does it look like?

Metal clip device. Dr. Welker's answer displays a picture of one type of aneurysm clip, which is commonly utilized in the brain. a aneurysm clip is a small piece of metal that is applied to a blood vessel to close it off / seal it off in order to prevent it from receiving further blood flow. When the metal clip is applied to the vessel, it permanently occludes the blood flow. This keeps aneurysm from bleeding.
Preventing rupture. A brain aneurysm that is at risk off rupture, or has already ruptured, can be clipped. The clips are available in many shapes ANS sizes, here's a picture of one in someone's brain.