Tar is a very thick liquid that's used to cover road surfaces. You can't walk on fresh, hot tar—you have to wait until it's completely dry.
Besides coating streets and parking lots, tar is also used to waterproof things like roofing shingles and wooden boats. Because it has disinfecting properties, you can sometimes find tar in soap, medicinal shampoos, and even as flavoring in candy and alcohol. When you tar something, you spread tar on it. Long ago, to tar and feather someone was to apply painfully hot, burning tar and feathers to their skin, as a brutal kind of vigilante punishment.