To bob is to dip up and down, the way something does when it floats on the surface of water. A lobster buoy bobs in the ocean, marking the spot where a lobsterman's trap is set.
Your friend stands out when her red hat bobs among a crowd of people, and so does a red boat as it bobs on a lake. You might bob your head to indicate agreement or get a short haircut that's also called a bob. The motion bob comes from the Old English bobben, "to strike," while the hair style stems from the earlier bobbe, or "cluster," and first referred a trimmed horse's tail.