A small foam pad with Velcro straps used to cover the sharp steering stem of a BMX bicycle, thus hopefully preventing ball-busting injury in the event of a crash. Popularized in the 1980's, they have fallen out of favor in more recent years secondary to the fact that they are not considered cool, and they don't really work anyway.
Hey dude...that's a rockin' nard guard you got there. Now all you need are some elbow pads and training wheels and you'll be ready for Pee Wee's Playhouse!