When a person focuses too much on small detail (trees), and misses the big picture (forest).
Stressed out person on vacation: "we have to awake at 4:00am, eat breakfast at 5:00am, goto the tour at 6:00am, arrive at the park 1 hour later, buy this, see that, go here, go there... are you writing this down!"

me: "how about, I'm on vacation, I'll be resting and meet you for lunch."

me behind the other person's back: "yikes, that person Can't see the forest through the trees."
by johncoleman83 March 17, 2017
This term means someone is so simple minded that they can't figure out the simplest problems. Their minds are like great big empty voids of any kind of logical thought. This is why even though the can see the trees their simple minds can't grasp that's the forest.
Ray : Did you see that guy trying to change his flat tire, it took him an at least an hour.

Tony :That guy can't see the forest through the trees.
by TV CAR March 30, 2010