Whispering into someone's ear; speaking softly close to someone's ear to tell a secret or give a quiet message.
彼女は耳打ちして秘密を教えてくれた。
She whispered a secret to me.
会議中、隣の同僚に耳打ちで要点を伝えた。
During the meeting, I whispered the main points to the colleague next to me.
演出前に演出家が俳優に耳打ちした。
Before the performance, the director whispered to the actor.