Just like English，two negatives resolving to a positive. For example： “我不是不同意I do not disagree” could mean “我同意 I agree（not mean I quite agree）"。 这不是不可能。It's not impossible,could mean 这是可能的 It's possible（does not mean that there is a great possibility.） 他们不得不离开这，,could mean 他们一定得（děi）离开这（It is not their will） It is my understanding,I hope to help you.