To espy something is to see it, or glimpse it. If you look through binoculars long enough, you might espy a colorful bird.
Though you can use the verb espy whenever you want to say "see," "spot," or "notice," it's primarily a literary word that you'll mostly find in books. One character might espy another strolling in a garden, or a pipe-smoking detective might espy a clue behind a parlor door. Espy comes from the Old French espiier, "observe, watch or spy on," from a Germanic root.