Gohan’s arm was injured during the Cell Games when he protected Vegeta from one of Cell’s attacks.
In the anime Dragon Ball Z, during the Cell Games saga, Gohan, who is the son of Goku, faces off against the villain Cell. Gohan ascends to Super Saiyan 2 and becomes the only one capable of defeating Cell. However, during the climax of their battle, Vegeta, in a rare moment of heroism, attacks Cell to distract him and help Gohan. Cell retaliates, and Gohan jumps in front of Vegeta to save him, resulting in his left arm being severely injured by Cell’s energy blast.
This injury is significant as it leads to one of the most memorable moments in the series. Despite his injury, Gohan musters the strength to face Cell with the help of Goku’s spirit and the remaining Z Fighters. With his right arm, Gohan launches a one-handed Kamehameha wave against Cell’s own. The two powers clash, but with the encouragement of his father and the support of his friends, Gohan overpowers Cell, destroying him once and for all. The injury to his arm thus becomes a symbol of Gohan’s bravery and sacrifice.