Like many parts of the northern hemisphere, the leaves are just starting to change colour in my area.
This autumn-related phrase is a well-known Japanese 4-character idiom.
一 = one
葉 = leaf
知 = to know
秋 = autumn
This phrase is about understanding the essence, changes, or decline of things from noticing slight signs or phenomena.
Such as realising the arrival of autumn by observing a single leaf fall.
