The Phantom Zone is a prison dimension primarily associated with Superman as it's the place he can banish his most dangerous enemies.
The Phantom Zone was first seen in the episode "Devil's Snare" after the Justice League wrongfully accused Harley's Crew for the chaos in Gotham City and decide to banish them to the Phantom Zone. Superman used his Phantom Zone projector to create a portal, however, before they all got pulled through Poison Ivy managed to grab onto Wonder Woman's magic Lasso of Truth, proving their innocence.
It was seen again in "Lovers' Quarrel" when the League attempted to send Ivy there after she was mind-controlled by Doctor Psycho as part of his plan to take over Gotham and they deemed her too dangerous to keep around. However, Harley threw herself between them, exclaiming she wouldn't let them kill her and they explained their intentions to put her in the Phantom Zone, she asked to be sent with her. However, before anyone could be transported, Psycho used his control over Ivy to infect the Justice League members with her pheromones, effectively distracting them.