used as a verb, "bombing hills" is the act of riding down a hill on a longboard (a downhill longboard, not a surfing longboard). You can do this in any number of ways (see longboard for examples), and no matter how you do it, it's pure fun.

verb. "Bombing hills" is the act of riding a downhill longboard down a hill. It can be used to mean any sort of riding, but specifically it means "to ride as fast as possible to the bottom."