|“||Sam: We're collecting souls to build a bomb.
Dean: To blow the Darkness to Hell.
|“||Castiel: How many souls are in there?
Billie: Couple hundred... thousand. I raided the Veil.
The Soul Bomb was an extremely destructive weapon that uses Light, fueled by the power of over a couple hundred thousand souls, designed to destroy the Darkness. Because of its use, it is arguably the most destructive weapon ever created.
The bomb's power depends on the amount of souls inside, and the crystal used can take in at least 200,000 souls. Rowena MacLeod was able to put all the souls inside Dean Winchester's heart; however, Dean would only be able to contain it for about an hour before it exploded on its own.
To activate the bomb once inside himself, Dean needed only to press his thumb and index fingers together. How devastating the explosion would have been was never shown, though it would certainly have cost Dean his life. Ultimately, Dean did not use the soul bomb, instead convincing the Darkness to reconcile with God. After the Darkness healed God, God removed the souls from Dean by absorbing the bomb before departing the Earth.
Dean later discussed getting Rowena to make another Soul Bomb in hopes that it would work against Jack Kline, but this never came about.
After God unleashed all of the souls of Hell upon the Earth, the Winchesters discussed with Rowena the Soul Bomb and asked her to make something similar to hold all of the unleashed ghosts. Rowena admitted that it took all she had to create it the first time and wasn't sure about a second attempt. Rowena eventually managed to create a weaker variant of the Soul Bomb, the Soul Catcher.