They mean greener quite literally. Satellite imagery is spectrally analyzed and more of the light reflected from the earth's surface is in a frequency range considered "green." It doesn't mean ecologically friendly. Replacing rainforest with oil palm monoculture is extremely destructive ecologically, but oil palms are just as green as any other tree.

Also, if they're analyzing water color as well, large-scale algal blooms can conceivably result in a greener surface, but they're a pretty bad thing because they suck up too much oxygen too quickly out of the water and kill everything else.