When an element such as plutonium or uranium hits neutrons, an atomic nucleus is split into two fragments and release fission energy. A single nuclear fission can only release a little energy but neutrons released in nuclear fission produce an additional fission with another nucleus. This nuclear chain reaction releases a gigantic energy (heat rays, atomic blast, radiation). Atomic bomb uses this energy as a weapon.
In order to explode an atomic bomb, a certain amount of fissile material, called critical mass, is necessary. The fissile material used in Hiroshima was uranium-235. Inside the bomb, uranium-235 was divided into two pieces, both of which are below critical mass. Using an explosive device, the two uranium-235 were brought together to form a critical mass that exploded instantaneously. This is known as a gun-barrel-type bomb.
It was nick-named "fat man"
It was nick-named "little boy"