“He is not able to see the wood for the trees.”

It means that he is not able to see the wood or the forest, as he pays a lot of attention to the individual trees. When you pay attention to individual trees, you miss the beauty of the forest as a whole.
When a person pays a lot of attention to details, he will not be able to see the overall picture. Anyone who gets lost in details will not be able to see the whole patters. In such contexts, you use this idiom. Too much attention to details will make one forget the main thing. When you get lost in details, you lose the view of the whole.