This idiom is used to describe people who have high standards for certain things and this can have different nuances depending on the situation. For example, it might mean people who want high quality things in life such as their job, spouse, house or things in general. And so, it can even be interpreted as ‘snobby’ or ‘picky’. Or it can refer to people who have good tastes. If you translate this idiom literally, it would mean "One's eyes are high."