This is the 32nd most frequent Korean word.
것을
"것을" means "the thing that" or "what" (object form).
'것을' in this sentence represents an object that has been heard by 'me', making it the object of the verb.
나는 그것을 들었다.
I heard that.
'것을' here refers to 'something new' as the object of the action 'learned'.
박물관에서 새로운 것을 배웠어요.
I learned something new at the museum.
'것을' indicates 'the act of breaking promises' as the object disliked by 'her'.