This poem is not a riddle, speaking with the voice of some mysterious "I" until the end, where the reader is shocked to find out that it's a mirror, and not a person speaking. Instead, the poem lets us know from the start that we're hearing from a mirror, with its title, "Mirror," and its first line, "I am silver and exact."
The first stanza describes the mirror, which seems to be like one of those people who doesn't tell white lies – it's truthful and exact, but not cruel.
As the first stanza personifies the mirror, showing us some of its human characteristics, we also find out a little about the mirror's life. Most of the time, it reflects a pink speckled wall, which could be found in any bathroom, but it also sees a lot of faces, and a lot of darkness.
Jump into the second stanza, and the stakes have changed. The mirror is no longer a mirror, but a lake, which also shows reflections. And we get to see a whole new character: a woman. We saw faces in the first stanza, but now we focus on one face in particular.
This woman, we find out, isn't very happy with her reflection in the lake, so she tries to find a kinder reflection under the light of a candle or the moon. When the lake reflects her faithfully anyway, she cries and gets upset.
In the last two lines of this poem, we see why this woman is so upset: in her watery reflection, her past is drowning, and a horrible future is rising to meet her.