Tracer ammunition (tracers) are bullets or cannon caliber projectiles that are built with a small pyrotechnic charge in their base.
Almost all of the bullets fired in Fury are tracers. This inspired some guffaws from audiences, who claimed that the bullets looked like lasers from the Star Wars movies. Unbeknownst to such moviegoers, the tracer fire is completely accurate, and actually is what inspired such lasers seen in science-fiction movies.
The chemicals painted on the tip of the bullet are ignited by the burning powder and burn very brightly, making the projectile trajectory visible to the naked eye. This enables the shooter to make aiming corrections without observing the impact of the rounds fired and without using the sights of the weapon.
When used, tracers are usually loaded as every fifth round in machine gun belts, referred to as four-to-one tracer. This was shown in Fury, when Trini Garcia confirmed this. Platoon and squad leaders will sometimes load their magazines entirely with tracers to mark targets for their soldiers to fire on. Tracers are also sometimes placed two or three rounds from the bottom of magazines to alert the shooter that their weapon is almost empty.
Tracer rounds may also ignite flammable substances on contact from a nominal distance. This is correctly displayed in Fury, when some Nazi soldiers are shown with bright flames due to tracer fire.