A string line (also called a builder's line) is a length of strong nylon line pulled tight between two stakes or posts. The tight line marks a straight edge: where a fence runs, how high a brick course should sit, or where a row of gate posts must line up.
A tight string line is straighter than the eye can perceive over distances up to 30 m. For longer runs you need a laser line or a series of short strings, because the line itself starts to sag under its own weight.
Gate-trade use: setting out the line of a fence run, lining up multiple post tops to a level, marking the gate-leaf edge for a square installation. Always pull the line tight enough to twang.