"She walks in beauty, like the night Of cloudless climes and starry skies; And all that’s best of dark and bright Meet in her aspect and her eyes; Thus mellowed to that tender light Which heaven to gaudy day denies."
This stanza is about a rare and exceptional kind of beauty. Byron is trying to communicate a certain perfection of beauty, and he turns to a peculiar feature of the night sky to explain what he means. On a clear night (that's what he means by "cloudless climes"), the stars can be so bright as to light up the darkness, but in a "mellow," subtle way—not the kind of overly bright, "gaudy" sunlight of the daytime. For Byron, the starlight is perfect—it brings together "all that's best of dark and bright." In that balance, Byron sees perfect beauty, and he says that the subject of his poem (the "she" he keeps talking about) is as beautiful as that particular kind of rare, perfect, "tender light."
