When constructing a structure, plumb means perfectly vertical when measured with a plumb bob or a level. A plumb object is responsible for maximizing the force of gravity by transferring the load straight down to the ground or grounding it. Any deviation from the plumb itself weakens that force.
This is especially important when framing windows or doors or setting posts or beams. A vertical framing member such as a stud or a post becomes a plumb when it is perfectly straight and when the horizontal member holding it in place is level or has no tilt. A plumb bob is a tool used by carpenters, usually made of brass that has a cone shape that tapers to a sharp point. A plumb bob is a more reliable tool then a level because the vertical direction from the plumb bob doesn’t have to touch anything else and relies only on gravity alone.