The speaker is describing someone who feels essential, the missing piece that makes other things work. Each line pairs her with something that only means anything because of what it needs: a canvas gives art somewhere to live, a violin is silent without its strings, a candle is just wax until there is a flame. She is the thing that brings each of them to life.
Some of the images are less obvious. "Words to the letters not posted" suggests feelings written down but never sent, and "love to the unloved" gives her an almost healing role. The line about the mess on the palette is a nice touch too, since it finds beauty in something untidy.
The closing line, "She is surreal," steps back from all the comparisons. She seems too wonderful to be quite real, almost dreamlike. The mood is tender and admiring, a little awestruck, as if the speaker is running out of comparisons and can only land on that one word.