The tiniest transistors have trillions of atoms. Nanotechnology would let us arrange and rearrange things atom by atom. A nanocomputer, for example, could be millions of times faster than that PC on your desk, and so small you'd need a microscope to see it. In theory, nanotechnology could cure cancer, clean the environment, restore extinct species. Or it could destroy us.
Nanotechnology is an infant.
Joy thinks it will grow up fast. He's also concerned about our motives in this quest for knowledge. The details change, but the fundamental ways in which we threaten ourselves remain the same. Devotion to self. Ignorance of others.