This is a great poem by Sylvia Plath, and one that addresses in a very unique way the subject of ageing that seems to preoccupy so many people nowadays. The image that your question refers to comes in the second stanza when the mirror reports that the woman who is always looking at herself in the mirror will often turn away and use other objects instead of the mirror to view herself. Note what the mirror tells us:
A woman bends over me.
Searching my reaches for what she really is.
Then she turns to those liars, the candles or the moon.
Faced with the "exact" representation of herself that the mirror "faithfully" gives, the woman cannot accept her ageing, and so she turns to other sources of light such as candles or the moon so that her reflection will not be so precise and harsh, softening the wrinkles and her ageing appearance. It is very interesting to remember though that in this quote the mirror refers to the candles and the moon as "liars," which seems to contradict its earlier statement that it is so objective and sees everything "unmisted by love or dislike." The mirror is perhaps not so impartial as it seems to present itself after all.