It means that if you look at things one at a time, you might not realize that a branch of separate "trees" go togehter to make a "forest"
When you are too close to a situation you need to step back and get a little perspective. When you do you will notice there was a whole forest you couldn't see before because you were too close, and focusing on the trees.
Simply that you have focused on the many details and have failed to see the overall view, impression or key point.
Example "can't see the forest for the trees"
He is so caught up in her affectation that he " cant see the forest for the trees"
He is to busy on the outside looking in that he cant see the forest for the trees.