Literally: summer – insect – doubt – ice
Alternately: A person of extremely limited experience, knowledge, and discernment. Someone ignorant, doubtful, or distrustful of everything that falls outside of their own (meager) personal experience. Like an insect that is born and dies all within the span of a single summer, which does not believe that winter ice even exists.
Notes: For historical reasons, this may also be read as 夏虫氷を疑う (natsu mushi koori wo utagau); “ice” may also be rendered with the rare/archaic character 冰 without any change in meaning or pronunciation.
This reportedly comes to us from foundational Taoist philosopher Zhuang Zhou (荘子, Japanese Sou Shi), aka Zhuangzi, found in the “Autumn Floods” (秋水, Shuusui) section of the eponymous Zhuangzi text.
There are also longer kotowaza variations on this theme, including the cute image of 夏の虫冰を笑う (natsu no mushi koori wo warau), “the summer insect laughs at (the idea of) ice.”