A crank is a lever attached to a rotating shaft. Sometimes the crank is used to turn a shaft – for instance when it is used as a handle, and sometimes the crank is driven by the rotating shaft. In this case, the crank can be used to change the type of motion.
A crank can convert a rotating motion into a reciprocating motion. This means that if you attach a rod to the central pin of the crank below, the rod will go up and down.
See the Geneva Wheel working.