It implies that someone can see details that others miss, and that their observation skills are highly developed.
Here are some examples of how it can be used:
* "The detective had an eagle eye for detail, spotting the tiny bloodstain on the carpet that others had missed."
* "The artist's eagle eye caught the subtle shades of color in the sunset."
* "With an eagle eye, the hunter spotted the deer hiding in the brush."
The idiom draws its meaning from the real-life eagle, which is known for its incredible eyesight.