Hand gesture used by motorcyclists and some others in respect and acknowledgment of fellow riders. An extended left hand wave with the arm slightly below the handlebar. This is done for both safety (don't wanna take your hands too far off the bars) and discreteness (it's an "inside thing".) Most riders are very friendly this way. Others won't return the gesture if you're riding a "foreign" bike or one not of the same make as theirs. Harley riders are notorious for this and other jackassery.
Hey, Dude! Low Five as we pass opposite ways on the road.