This is a poem that presents us with the speaker looking at the stars at night only to be struck with complete awe at their beauty and majesty. Focus on the way in which the stars are described. The speaker uses metaphors to compare them to precious stones, obviously emphasising both the way they emit light and also their unestimable value, describing them as "White and topaz, / And misty red." The poem then continues to imbue these stars with life as they are described as "Myriads with beating / Hearts of fire" and are said to be so old and ancient that even aeons are unable to "Vex or tire" them. As the speaker watches their "stately and still" progess up the dome of heaven, her response is one of complete awe as she is forced to contemplate the beauty of the stars and also is forced to evaluate her own smallness in the light of their majesty:
And I know that I
Am honored to be
Of so much majesty.
Notice the way in which the word "Witness" is given a line to itself, clearly indicating that it is important. This poem, like so many other poems by Sarah Teasdale, shows the beauty of nature in our world if we have eyes to see it. The implication of this poem is that we are all able to be a "Witness" of such staggering sights, if we change the way that we look at nature and can therefore open ourselves to see its majesty.
I think the meaning would be that stars are surrounding the sky and heaven, making us look so small and insignificant; they can take us by awe due to their splendor and elegance shown by their light and stillness among us in the sky.
I hope I helped some! =D