Caltrops is an offensive skill that when activated you throw spikes in front of yourself. When another player touches one, they lose 10 health. Once a spike hits someone, or after a set time period, it will disappear. Caltrops are useful to hit multiple people at a time with control, but there may be better skills than this one, depending on the situation.
Leveling up the skill will increase the spike count thrown, and decrease the cooldown you need to wait for after you use the skill.