I am silver and exact. I have no preconceptions.
The speaker of this poem is a mirror. It is saying that is does not have a preconceived idea of what its looking at, and it will take in and reflect exactly what it sees.
What the mirror reflects is not affected by how much it hates or loves you. The mirror reflects the truth.
This could also reflect Plath’s numbness that resulted from her severe depression: she became unable to feel anything, let alone love or hate anything.