|Metaphor is one of the most fundamental figures of speech. Much of our everyday language is metaphorical in nature; we just don't always recognize the implicit comparisons that our most common expressions are making. A metaphor is a comparison of unlike things that seeks to identify the shared qualities of the things. |
The critic I.A. Richards has provided a useful set of terms to describe the parts of a metaphor. The tenor is the "gist" or "point" of the metaphor, the phenomenon the metaphorical turn of phrase is trying to convey. The vehicle is the concept conveyed by the word or group of words that is used to make the explicit comparison.
The trees are being compared here to a shield, protecting the pool and contributing to the overall characteristics of the placid locus amoenus.