This transformation skill is only available for Saiyan characters. The earliest you can get this skill is in PQ #14: Legendary Super Saiyan.
To increase the chances of Goku transforming to a Super Saiyan during the mission, defeat Piccolo and Vegeta first. Once Goku and Krillin arrives, defeat Krillin first to push Goku to transform to a Super Saiyan. Defeat him while in this form to get a higher rank.
Once you got the skill, place it in one of your Ultimate Attack slots. It needs 300 Ki to activate and once activated, your Ki will be constantly drained. You can use your Super Attacks and Ultimate Attacks continuously without consuming Ki, which allows you to increase your damage output, especially during boss fights. You can even use your finishers even when below one Ki bar, as long as your Super Saiyan form is active and you have Ki left.
Also, once your Ki is trained because of your Super Saiyan transformation, your whole stamina bar will reset, regardless of how much Stamina you have stored. This makes you vulnerable after ending your transformation. As such, it is recommended to use an evasive skill that can stagger or paralyze an opponent, then follow up with an Ultimate attack up close when your Ki is about to be drained. If done correctly, your stamina will reset during the animation your character does for his/her ultimate attack, giving you temporary invulnerability while recovering your stamina at the same time.