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It was taught to Goku by North Kaio while training with him in the afterlife. Basically, the user gathers a huge amount of energy from all living things, and transmits that energy into a huge sphere of incredible destructive power. At first Goku only assembles the energy from non-human life (like plants and animals). While fighting Freeza (Frieza), he collects energy from the Namekian solar system. Even though it was powerful, it still wasn't strong enough to defeat Freeza.
The spirit bomb's energy was greatly increased after Vegeta made a suggestion to Goku while fighting Kid Buu. He suggested that they have the earthlings give Goku their energy for the bomb. This succeeded in making the spirit bomb powerful enough to defeat Kid Buu. Goku used a similar method to create a spirit bomb to destroy Omega Shenron. Except in this case he gathered energy from every ally in the universe (with the help of King Kai).
Additionally, the user must be pure of heart to manipulate and gather the energy, otherwise the attack will backfire on them and hurt or even kill them. Krillin reveals during the Dragonball Z feature: Super Android 13 that Goku cannot gather the energy while in his Super Saiyan state. This is because the state of Super Saiyan is inherintly malicious. In the feature, Goku manages to channel the spirit bomb energy into himself while he is in his Super Saiyan form.
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