I'm guessing you're talking about the atomic bomb that ended WWII?
Well, for starters, the president decided to drop it because he didn't want our soliders to have to go to war and have the risk of them dying.
He thought it would be a quick way to end the war, and a good way to show other countries how powerful we were at the time. The Atomic bomb explodes because of nuclear fussion..when each atom keeps exploding except it happens in seconds causing a major disaster. The bomb was dropped on Hiroshima and Nagisaki (islands in Japan) causing the whole city to be in complete ruins. Most people died, but survivors had permanent burns and you can still see people there today who have suffered from the explosion. Well, I hope that helped a little! =D