The story
Komorebi is written in Japanese as 木漏れ日, with the characters for tree, for leaking or seeping through, and for sun. Read literally it says something like 'tree-leak sunlight'. Japanese dictionaries define it as sunlight that comes through the leaves of trees. It is an everyday word, found in song lyrics and in descriptions of walking routes.
English knows the effect well and describes it in phrases: dappled light, sun through the leaves, sunbeams in a wood. What English lacks is a noun for the light itself, the bright patches on the ground or on your arm. Komorebi lets you point at it the way you point at a rainbow.
The pronunciation hides a small rule of Japanese: hi, 'sun', becomes bi when it sits at the end of a compound, a sound change called rendaku. That is why it is komorebi and not komorehi.
A scene: a warm June afternoon in a beech wood. The wind moves the canopy, and round spots of light slide back and forth across the path. Anyone who stops to watch them is watching komorebi.
The word became known outside Japan through lists of words other languages lack. In 2023 a film added to it. In Wim Wenders' Perfect Days, a Tokyo toilet cleaner photographs the light through the treetops every lunchtime, and the film closes with a short explanation of the word.