You'd think with a title that snazzy Le Guin would be referencing some famous poem or play, probably something by Shakespeare or Blake. But you'd be wrong. Le Guin does reference a now-famous poem, but it's one she wrote for this very novel.
The title comes from the poem recited by Estraven while Ai and he traversed the Ice. The poem comes from the Handdara religion. We've included it in full here:
Light is the left hand of darkness
and darkness the right hand of light.
Two are one, life and death, lying
together like lovers in kemmer,
like hands joined together,
like the end and the way. (16.44)
At the most basic level, the title is meant to draw your attention to the poem. It's saying, "Hey, pal, pay attention to this. This stuff is important! Whip out your highlighter, or at least put a little star by it."
But why is this poem so important that it gets recognition in the title of the entire book?
Notice that, directly after the poem, Estraven and Ai talk about dualism. Dualism is the idea that states exist in two parts opposite each other—things like man/woman, light/dark, good/bad. Usually in dualism, one thing is traditionally viewed as superior (man, light, good) and the other inferior (woman, dark, bad).
Note: we used the word traditionally as we know very well women are not inferior to men and dark people are not inferior to light people. You can delete your hate mail now.
The poem speaks of unity: light and dark joined together like two hands. Unity of opposites is a hugely important theme in the book. The Gethenians are both man and woman. Light and shadow are needed to cross the Ice safely. And Ai's mission for the Ekumen will unite Orgoreyn and Karhide if successful.
So, the title points the reader to pay attention to this poem because the poem expresses the super-ultra-mega important theme of unity over dualism. And that, Shmooper's, is what's up with the title.