Meaning- to overlook the overall situation because of a focus on small details.
Usage- During his writing he focuses on grammar but forgets the content. He can’t see the forest for the trees.
Origin- This idiom,which has been popular for many years,created this picture in the mind of the writer who invented it: a person in the country focuses so intently on each individual tree, leaf, branch, twig ,that she misses the splendour of the huge forest she’s in . Afterwards,if someone asks her,”How dis you like the forest?” she might answer- ” What forest? All I saw were some trees”. This saying means to be unable to understand the whole situation because you pay too much attention to the small parts.