"You can't see the forest for the trees" is an idiom in the English language used to describe a case in which one is so focused on the details of a given project that he cannot see or understand the whole issue. An idiom is a commonly used figurative expression that differs from the literal meaning of the expression. Literally, an individual in a forest may be so interested in individual trees that he ceases to care that he is in a forest. In general, this is not a pressing concern. In common use, however, an individual who cannot see the forest for the trees is so fixated on individual details, or "trees," that he completely loses sight of the overall issue, or "the forest."
The idiom "can't see the forest for the trees" can be used in a variety of ways. Individuals working on a particular project sometimes become so engrossed on a particular detail that they lose sight of the overall project. Someone might tell such people that they can't see the forest for the trees in order to remind them that the small detail is insignificant in light of the overall project and that they should move on. Similarly, students attempting to grasp complex subjects may become so hung up on particular details that they lose sight of the overall concepts they are trying to understand.
In a more philosophical sense, "can't see the forest for the trees" is sometimes used to describe one's overall approach to life. Some people become so fixated on particular aspects of their careers or other concerns that they lose sight of the overall significance of life. When someone tells such people that they cannot see the forest for the trees, the "trees" represent the particular life details that take up too much attention while the "forest" is life as a whole with its many intricacies and possible experiences. By focusing on just a few trees, one misses out on the many opportunities offered by the whole forest.
The particular expression "can't see the forest for the trees" can vary somewhat. One may, for instance, say "can't see the forest through the trees," and the meaning will still be understood. In British English, however, it is common to say "can't see the wood for the trees." These different formulations of the idiom are all intended to convey the same general meaning.