What is a plumb bob?

A plumb-bob or a plummet is a weight with a pointed tip on the bottom that is suspended from a string and used as a reference line that is perpendicular to the ground.

This instrument has been used since the time of the ancient Egyptians by bricklayers,masons, and carpenters to ensure that their constructions are "square", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are included in the kits of various instruments such as levels and theodolites. They are used to set the instrument exactly over a fixed datum marker, prior to taking fresh readings. 

Thank you to www.wikipedia.org for the definition and picture used in this answer.