Snare is a Spell added by Roots. It is created with the base ingredient, a Cobweb, a Spider Eye and a Cactus. When cast, hit mobs will gain Slowness IV for 2 + 2 * the potency seconds.
— Runic Tablet
By incorporating a cobweb into your spells, you gain the ability to ensnare targets. Using the above recipe, you can create a spell that will significantly slow entities in an area around the spell.