The implosion type atom bomb is a simple design that uses explosives to compress a ball of uranium to a point where the density of neutrons produced by spontaneous decay is enough to cause a chain reaction, hence explode the bomb. The implosion method can use either uranium or plutonium as fuel.
The nuclear fuel is shaped into a sphere – the 'pit' in the diagram, surrounded by conventional explosives. When these are detonated the force of the explosion squeezes the pit into a supercritical mass long enough for uranium to explode. The shock wave that compresses it must be precisely spherical, otherwise the fuel will escape out through a weak point. To create the necessary explosive force in a perfect sphere, shaped explosive charges must be detonated at exactly the same time.