To discern an overall pattern from a mass of detail; to see the big picture, or the broader, more general situation.

Usage notes

• This is almost always used in negative constructions, often starting with can't, as it is a negative polarity item.
• The portion "forest for the trees" may seem grammatically nonsensical to modern speakers and learners who are not familiar with, or expecting, the Old English meaning of for (especially outside of an Old English context). This older usage of for means "because of" or "due to", also found in "for want of a nail". The idiom may be more readily parsed today in the form [can't] see the forest, but for the trees.

Alternate Forms

• Lose the forest for the trees

Sources

Wiktionary