As candy is pleasing to the taste perception, eye candy is pleasing to the sight perception.
Generally it is anything out of the ordinary that makes a setting more visually appealing, attractive and compelling. Elevations, embellishments (waves, waterfalls, flowers, stones), colours of light and terrain, textures, custom buildings and placement, layout of towns, and atmospheric effects are all eye candy tools for the designer. Take any random map and compare it with images in the eye candy thread and you'll get a sense of the difference. Many people attribute good scenario design with effective use of eye candy.
One caution though- and I am sure some people will object to this statement- balance eye candy with playability. While I love eye candy, it must in general serve the story, and too much of it sometimes slows game play. If you enjoy pure eye candy, best to design things for screen shots. But checking out some of the nicer scenarios/campaigns in the DL section will show you how good designers balance the two. When making scenarios playable for a wide variety of computers, some balance and discretion must be exercised.