An atomic bomb is the name given to a device designed to produce an enormous explosion by harnessing nuclear energy.
The United States was the first nation to produce atomic bombs. So far, two bombs the United States dropped on the Japanese cities of Hiroshima and Nagasaki, in the closing days of World War 2, have been the only military uses of atomic weapons.
The first atomic bombs, like the two dropped on Japan, relied on producing a chain reaction of fissioning radioactive nuclei. Atoms of the heaviest elements found in nature, like Uranium and Thorium, have no stable isotopes. All isotopes are radioactive.
The radioactive isotopes of Uranium and Thorium have relatively long half-lives, measured in the millions of years. However, as Leo Szilard was the first to note, an atom of Uranium 235 could be stimulated to fission, if it was struck by a slow neutron. This fission of the Uranium nuclei into two smaller atoms releases a very large amount of energy - and another neutron. Szilard suggested that, if a large enough mass of relatively pure Uranium 235, were gathered in a single place, neutrons released by the fission of some atoms would stimulate the fissioning of other atoms. He called this a "critical mass".
He described how, if this fissioning could be controlled, it could be an extremely useful source of energy. The mechanism for harnessing this kind of energy is now called a nuclear reactor.
And, when a critical mass is brought together suddenly, forcing a very large fraction of the unstable radioactive nuclei to fission, almost simultaneously, it would result in a massive explosion.