What is a chalk line?
A chalk line (also called a "chalk reel" or "chalk box") is a simple hand tool that consists of a box with a reel of string inside it. The box is filled with chalk powder and the string (once coated in chalk) is reeled out and used to impart straight lines onto surfaces.
On the end of the string is a metal hook which can be attached to an object. The chalk-coated string is reeled out across the surface and pulled taut. Once the string is in the correct position, it can be 'plucked' at its centre and the tension in the string forces it to "snap" back down and impart a chalk line on the surface of your object. This is called "snapping the line".
A brief history of the chalk line
Chalk lines have been used since the ancient Egyptian times. Stone masons would use them to mark out straight lines prior to cutting the stones. Since then, builders from every culture around the world have used chalk lines for everything, from marking ground layouts to hanging wallpaper.
Other countries use ink lines instead of chalk lines. Traditional ink lines are often called "Sumitsubo" and have a silken cord which runs through a combined ink-pot and reel and are often highly ornate and decorative.