A figure of speech in poetry is also known as literary devices or poetic devices. The main figure of speech used in the poem is one of personification.
Personification according to the eNotes site is:
a figure of speech in which abstractions, animals, ideas, and inanimate objects are endowed with human form, character, traits, or sensibilities.
What this means is that non-human things are given traits that only humans typically have. An example of personification would be "the wind is laughing" or "the clouds are screaming". Wind cannot laugh and clouds cannot scream. Only people can laugh and scream.
The entire poem "Mirror", by Sylvia Plath" is a personification poem. The personification is denoted by the title of the poem. Without the title, one could justifiably assume that the poem is about a person given the human characteristics depicted in the poem itself.
Whatever I see, I swallow immediately.
Here, the line refers to a mirror swallowing the reflection seen in the glass. Mirrors cannot swallow anything, but people can. Therefore, this examples the personification used in the poem.
Another figure of speech that exists in the poem is one of hyperbolic language. A hyperbole, as defined by eNotes, is:
obvious and deliberate exaggeration or an extravagant statement.
Basically, a hyperbole is an over-exaggeration of the truth. Here, an example of hyperbolic language is:
Now I am a lake.
The mirror is stating (personification- mirrors cannot state anything) that it is a lake. This qualifies as a hyperbole because a mirror is nothing like a lake- it is small and has no depth (physically). The hyperbole exists because the mirror is describing how deeply one can look into a mirror and it would seem that the mirror is endless and deep.
The last example of figurative speech in the poem is one of a simile. A simile is a comparison between two things using the words "like" or "as".
Rises toward her day after day, like a terrible fish.
The comparison is the image of the woman reflected in the mirror to that of a fish.