Rounds where the base of the bullet has a chemical painted on the base that burns during flight so you can see where your rounds are going. Also very useful to the enemy, as it shows them where you are.
Tracer rounds usually start to burn at about 100-150 metres out from the firer. They are very useful for showing kids who like to play on rifle ranges where rounds REALLY go when they hit the "stop"-butt. You are also usually not allowed to use them on gallery ranges for precisely that reason.