'Once Upon a Time' Season 3, episode 16: The Wicked Witch reveals her master plan ... time travel?
Yes, time travel. You see, in "It's Not Easy Being Green," viewers got to see Zelena's backstory, and it mainly consisted of her finding out she was adopted and her dad never truly wanted her because her magic creeped him out. So she went to see the Wizard of Oz (who turned out to be Walsh, of all people!) who told her she was Cora's first born daughter, but since she couldn't help Cora become royal, Cora abandoned her. Regina was born next, and helped Cora achieve the royal status she yearned for, so she raised her without giving Zelena a second thought.
Even Rumpelstiltskin chose Regina over Zelena, since in order to enact his curse to find Baelfire in the land without magic, he needed a witch to give up what she loves most. Zelena loved Rumpel the most, and he didn't want to die (obviously), so he chose Regina. What's funny is all Rumpel had to do was tell Zelena that he wanted to travel to a different realm and she could have used her magic shoes to get there. But she was so angry that once again someone picked Regina over her that she said it was too late, and she vowed to destroy Regina.
Zelena's envy for Regina is what turned her green (literally green with envy), and she decided to find a potion/curse that will turn back time to give herself a second chance. She wants to go back to when Cora first abandoned her for a do-over. A.k.a. time travel.