The poem begins with a metaphor: the speaker says, "The Mushroom is the Elf of Plants." In other words, the mushroom is a unique and enchanting part of the plant world (though, technically, it's not a plant at all!).
An "Elf" is a mischievous little creature from folklore known for playing tricks and being elusive. The metaphor suggests that, like elves, mushrooms are small and capricious; they don't follow any predictable patterns, but rather pop up when and where they please.
Indeed, the speaker says that the mushroom is nowhere to be seen in the "Evening," but by "Morning," it appears "in a Truffled Hut." This is another metaphor playing on the way a mushroom looks: its cap forms a little "hut." The word "truffled" comes from a specific kind of mushroom, and this image also might make readers think of the mushroom as a kind of delightful little house in which an elf or fairy might live.
Anaphora in lines 2 and 3 ("At Evening"/"At Morning") highlights just how quickly the mushroom grows. Unlike many plants which take days or even weeks to rise from the ground, the mushroom seems to just appear overnight.
And notice the playful, bouncy sounds in line 4:
It stop opon a Spot
There's /s/ alliteration ("stop"/"Spot"), /aw/ assonance ("stop," "opon," "Spot"), and crisp /t/ and /p/ consonance. The line reads like a lighthearted tongue-twister, the poem's sounds helping to convey the mushroom's impish nature. The humorous sounds also create a fun, light-hearted tone—the speaker is enjoying musing on what makes the mushroom stand out from other plants!
This stanza also establishes the poem's form. Each stanza has four lines, making them quatrains. These quatrains also use something called common meter, a favorite of Dickinson's and also the meter often used for church hymns and ballads.
Common meter alternates between lines of iambic tetrameter and trimeter. An iamb is a poetic foot with two beats arranged in an unstressed-stressed pattern: da-DUM. Iambic tetrameter means there are four iambs per line, while trimeter means there are three. Here are lines 1-2:
The Mush- | room is | the Elf | of Plants—
At Eve- | ning, it | is not
Common measure also uses an ABCB rhyme scheme: every second and fourth line rhyme with each other ("not"/"Spot"). Using common measure lends the poem some familiar music.