Illuminating (usually incendiary) projectile used to designate an intended target. Tracer ammo is illegal for regular civilian use in the US as far as I know. I am not sure of the FFL regs.

I never liked the notion of using tracers because it gives away your own location as well as your intended targets. It is great stuff if you are in a dogfight and you need to estimate lead on arc to your opponent. It is also a useful tool for combat analysis in the field when static targets and marksmanship are not so easily discerned.

They are also a major fire hazard.