The sparks, nitzotzot, are the light that fell when the vessels broke and is now hidden in all things, even the lowest. Hasidism taught that every act, eating, working and speaking, done with holy intention, can raise the sparks it encounters. The image has parallels in the Gnostic and Manichaean myths of trapped light.
Nitzotzot
Sparks: the fragments of divine light scattered in the world, to be raised by holy action.
Also: holy sparks