I found it after a while looking. You feed him fish.
"Lord Jabu-Jabu resides in Zora's Fountain, where he is cared for by the Zora. After King Zora XVI refuses to give Ganondorf the Zora's Sapphire, he places a curse on Lord Jabu-Jabu, causing him to act strangely, and swallow his caretaker, Princess Ruto, while she was bringing him food, along with the Zora's Sapphire. Link must enter Jabu-Jabu's belly to rescue her and break the curse by defeating Barinade. In order to enter, Link must present a fish for Jabu-Jabu to open his mouth. The deity will then suck Link in. Afterward, Jabu-Jabu will simply open his mouth when Link approaches, allowing him to enter and leave as pleased."