What is meant by "Nafilah" in His—the Almighty's—saying: (And We bestowed upon him Isaac and Jacob as Nafilah)? And is it exclusive to Jacob only? If so, why were Isaac and Jacob conjoined?

The meaning of "nafalah" (supererogatory gift) is Jacob, because Allah gave Isaac to Abraham in response to his supplication, and then increased it with Jacob, and "nafalah" means an increase. It is also said that it refers to both Isaac and Jacob.