The "Violet" represents a person who is extremely talented but is afraid to show her talents. She could have been the most beautiful thing ever know to the mankind, only if she came out of her hiding and oyster shell, and showed her real talents. The poet is a person who recognized the qualities of ye violet, and loves them. He wants to show those qualities to the world, yet the violet wont leave her hiding.
It means one's hidden talents cannot be unseen by others.