A yellow sun grenade is a device developed by J'onn J'onzz to generate yellow sun radiation when detonated. The radiation is strong enough to temporarily give Supergirl her powers even when on a planet with a red sun.
When Kara Danvers and Mon-El were trapped on Slavers' Moon, its red sun Arcturus deprived them of their powers. Before Alex Danvers left Earth to rescue them, J'onn J'onzz gave her a single yellow sun grenade. She then led a D.E.O. rescue mission to the planet to retrieve the kidnapped human captives but during the escape, Izzy Williams was restrained by a Maaldorian soldier. Alex used the grenade to give Kara a dose of yellow sun radiation which restored her Kryptonian powers and allowed her to easily defeat the remaining guards and rescue Izzy.