A juniper berry is a seed cone produced by the various species of juniper plants. It's not a true berry, but is rather a seed cone with unusually fleshy and merged scales, which give it a berry-like appearance.
They have often used for food as a a seasoning, as well as a flavoring for gin. However, juniper berries also have a long history of being used as medicine, dating back to the ancient Greeks. As it is often used to fight bacterial infections, Doc Brown used distilled juniper berries to combat an infection in his dog Einstein's eyes, which he humorously referred to as cat-aracts, since the infection blurred Einstein's vision and, using a vision simulator, Marty noted that everything resembled cats.