He confessed to the mistake of his own accord, which impressed his manager.

of 是介词，表示所属关系或来源。 his 是形容词性物主代词，表示"他的"。 own 是形容词，强调"自己的"。 accord 是名词，意思是"意愿、同意"。 of his own accord 是一个固定短语，字面意思是"出于他自己的意愿"，引申为"自愿地、主动地"。