It refers to the act of resisting or opposing something, typically authority or a rule.
Here are some examples:
* The prisoners showed defiance in the face of their captors. (The noun "defiance" is the object of the verb "showed")
* Her defiance was evident in her every word. (The noun "defiance" is the subject of the sentence)
* He was punished for his defiance of the law. (The noun "defiance" is the object of the preposition "of")