Bundles of Pila are throwing spears that are used by tribes, but you can also start with a bundle of Pila with the 'Lost Tribe' Scenario. Despite the description possibly implying a finite amount of Pila, the Pila works like any other ranged weapon in the game in terms of durability and ammunition. With a long warmup time of more than 4 seconds and an equally long cooldown time, you will generally want to be careful with how you position your colonists equipped with the Pila as they'll be vulnerable for a long period of time after throwing.
Despite the lengthy periods, a Pilum that hits its target will generally hurt badly, and two torso shots can spell the end of the unlucky victim. A Pilum can also easily decapitate or incapacitate an unarmored foe too, but they are generally less effective at the upper limits of their range as both weapon and shooter accuracy drops off, so you generally want to keep behind solid cover at a distance to exploit these weaknesses.
However, this can become very difficult to micromanage as the game progresses due to sheer numbers of tribal enemies in the later game, meaning you could inevitably lose somebody or even multiple people if they aren't sufficiently protected against sharp damage.
Bundles of Pila (plural for Pilum) are throwing spears that were used by ancient Roman soldiers when swords were inadequate. The Pila in Rimworld appears to be based off of the thick variant of the Roman Pilum, which is the more recent and also lighter variant of the heavier thin Pilum.
The below graphs assume an unmodified shooter using a Normal quality weapon.