Instead of describing a palm tree directly, the poem gives us three quick, odd comparisons and lets them pile up. The palm becomes a navy, a sign from Holland, and a temple to a hen. None of these is explained, so the reader has to make the leaps.
Some of them are easier to follow than others. A navy of the afternoon suggests fronds swaying like ships' masts or sails, with the light and heat of the day as the sea. The Holland and hen images are more puzzling, and the poem seems happy to leave them that way. Judging by the playful logic, the speaker may be showing how a single object can set off a string of unrelated associations, some grand, some comic.
The mood is light, curious, and a little deadpan. The speaker doesn't tell us what he feels outright, but the delight in surprising images suggests he's enjoying the looking more than solving anything. The ending, with its small, humble hen, deflates the big "navy" and "temple" in a way that feels gently funny.