A string line (sometimes 'builder's line' or 'masonry line') is a length of strong nylon or polypropylene line stretched tight between two stakes, posts, or fixed reference points. The line marks a straight edge: where a fence runs, where the top of a brick course should be, where the front face of a row of gate posts must align.
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.