a remark that is added to a sentence, often to provide an explanation or extra information, that is separated from the main part of the sentence by commas, brackets, or dashes

The sentence "Her youngest sister - the one who lives in Australia - is coming over next summer" contains a parenthesis. “她妹妹——住在澳大利亚的那一个——夏天会来”这句话中包含一个插入语。

If, while you are talking, you say something in parenthesis, you say it as something extra and then continue with the main part of the sentence.

Of his origins he said very little, merely mentioning in parenthesis that his background was poor. 关于他的身世他谈得很少，只是附带地提到他家境贫寒。