I am silver and exact. I have no preconceptions.
Whatever I see, I swallow immediately.
Just as it is, unmisted by love or dislike
I am not cruel, only truthful –
The eye of a little god, four-cornered.
Most of the time I meditate on the opposite wall.
It is pink, with speckles. I have looked at it so long
I think it is a part of my heart. But it flickers.
Faces and darkness separate us over and over.
Now I am a lake. A woman bends over me.
Searching my reaches for what she really is.
Then she turns to those liars, the candles or the moon.
I see her back, and reflect it faithfully
She rewards me with tears and an agitation of hands.
I am important to her. She comes and goes.
Each morning it is her face that replaces the darkness.
In me she has drowned a young girl, and in me an old woman
Rises toward her day after day, like a terrible fish.
- Sylvia Plath
Can you tell it's a metaphor for language and poetry? I can't either. But if you just read it as written, it seems to be about a woman who has trouble dealing with ageing, and puts painful emphasis on her looks. I read Plath's diaries(a long time ago) and there is a section where she describes herself with the flu (runny nose, and such) and her surprise that her husband can still stand her because she is so ugly. (I am paraphrasing, and possibly misremembering.)
This is long, in-depth analysis of Mirror, by William Freedman. It's well worth reading, I'm sure, if you want to know more. He'll explain the language and poetry bit.