A chalk line is a piece of string or cord that is coated with chalk. The line is stretched tightly between two points and then snapped to release a chalky line onto the surface. A piece of string rubbed with chalk can be used for the odd occasion. But for frequent use, a mechanical self-chalking line is much more convenient.
A mechanical self-chalking line is a metal box containing a line on a reel. The box is filled with colored chalk powder. The line is automatically chalked each time it is pulled out of the box. Some models have a point on the end of the box so it can also be used as a plumb bob.