Tracer bullets are regular bullets spiked with magnesium, so that they burn hot and white so you can see them in the dark. If it’s dark and you’re shooting at something, you can’t see where your bullets are going, so you mix in some tracer bullets. They help you see where you’re pointed, so you can adjust your aim and get pointed in the right direction.
Being in the dark and not knowing where you’re pointed is uncomfortable and scary. The faster you can figure out how to mix some tracer bullets in, the sooner you can start feeling comfortable.