The nuclear test of the first detonation of a nuclear weapon was codenamed Trinity, but the atomic device was nicknamed The Gadget. The date of the Trinity test is usually considered to be the beginning of the Atomic Age. The gadget was an implosion-type plutonium device, similar in design to the Fat Man bomb used three weeks later in the atomic bombing of Nagasaki, Japan.
The term "Gadget" was a laboratory euphemism for the bomb, from which the laboratory's weapon physics division, "G Division", took its name in August 1944. At that time it did not refer specifically to the Trinity Test device as it had yet to be developed, but once it was, it became the laboratory code name. The Trinity Gadget was officially a Y-1561 device, as was the Fat Man used a few weeks later in the bombing of Nagasaki. The two were very similar, with only minor differences, the most obvious being the absence of fuzing and the external ballistic casing. The bombs were still under development, and small changes continued to be made to the Fat Man design.
The Gadget was an implosion device, which means the plutonium core is surrounded by many small explosives, these compress the plutonium and bring it closer to the point of causing it to go super critical. All those wires are attached to different explosives which burn at different frequencies. The trick of the 20 explosions is that they push the pieces of uranium (or plutonium) together to a ball with an over-critical mass, which explodes. They have to time this extremely accurately, however. Microseconds differences will make the ball lopsided and less effective. Part of the solution is to make each and every cable the same length which is why the Gadget looks like a ball of wires.