Answer:
The meaning:
This is a metaphor to express that flowers were so beautiful and vibrant that they seemed to be jewels gleaming under the sun
Step-by-step explanation:
Let's remember that metaphors are used to make comparison; a metaphor is a figure of speech in which a word or phrase is applied to an object to which it is not literally acceptable.
In this case, for example, flowers are not capable of gleaming, but the comparison with the shining jewels under the sun is made in order to express how beautiful, vibrant and shining they were.
Flowers were so vibrant it was like they were glistening under the light of the sun!