What Does Tigers in Autumn Mean in Chinese?

秋老虎 (qiū lǎohǔ) The Hottest Days in Autumn Key Learning Points (Preview):

热 (rè): adj. hot

害怕 (hàipà): v. to be afraid of

Why do people call it “秋老虎 (qiū lǎohǔ)?” It is a short hot period (7-15 days)  occurring in the latter part of …