the.com/tracer

means A tracer is a projectile loaded with a pyrotechnic charge that burns brightly in flight, letting the shooter visually track the bullet's path and adjust aim.

from From 'trace' (to follow or mark a path), itself from Old French 'tracier' and Latin 'tractus' — the pulled line left by something dragged. Military use picked up the word in the early 1900s, once armies began loading rounds with phosphorus or magnesium compounds that burned on exit — literally leaving a visible trace of the shot across the sky, so gunners could correct fire in real time.