At the center of every atom is a nucleus. Breaking that nucleus apart—or combining two nuclei together—can release large amounts of energy. Nuclear weapons use that energy to create an explosion.
Modern nuclear weapons work by combining chemical explosives, nuclear fission, and nuclear fusion. The explosives compress nuclear material, causing fission; the fission releases massive amounts of energy in the form of X-rays, which create the high temperature and pressure needed to ignite fusion.
Take a deep dive into nuclear weapons with the rest of this post, which includes accessible descriptions of fission and fusion, how nuclear weapons work, nuclear fuel, warheads in-depth, and more.