Snare is a trap from the Witcher 2.
It does, with the related alchemy skill at level 2, 40-100 damage to one foe, and can be gathered from the field once used. You will find many of them in the forest in chapter 1, and so shouldn't need to build any.
The fact that it does quite high damage, can be placed in packs, and can be gathered and reused makes it ideal to save money and kill monsters you're having a hard time to kill by the sword.