It's a slang term that just means to hit someone slightly or gently, but not maliciously, but to get their attention. Like if you were standing next to someone and they were looking the other way and you tap them on the arm a couple of times to get their attention because you want them to look in the other direction at something. To "pop someone" is a slang term for tapping someone on the arm or shoulder, or it could mean to hit them. "Casual" in this case just means slightly. A "knuckle butt" is a slang term for a clenched fist. If you make a fist and look at the crack between your fingers, it kind of looks like a butt.