Quivering means trembling. If your dog begins quivering with fear whenever someone knocks on your door, he's not much use as a guard dog.
When something's shaking gently, like a candle flame shivering slightly in a breeze, you can describe it as quivering. On a windy day, you might see the leaves of trees or the ruffled feathers of a bird quivering in the wind. There are a few theories about the origin of the word quivering including the possibility that the word quiver was developed onomatopoeically — or creating a word that sounds like its meaning, like a vibrating kind of shiver.