Of course, it is likely that Frost spent some time working on this poem, changing words to create the right mood, rhythm or structure. It is therefore unlikely to represent the thoughts running through his head directly. Nevertheless, the simplicity of the poetry can be viewed as what may have been running through the poet's mind, as he sat watching an evening unfold.
Fireflies in the Garden
by Robert Frost
Here come real stars to fill the upper skies,
And here on earth come emulating flies,
That though they never equal stars in size,
(And they were never really stars at heart)
Achieve at times a very star-like start.
Only, of course, they can't sustain the part.
I think it's a lovely poem, and a masterful representation of a mind at work, appreciating beauty, evaluating, comparing, making judgements about an experience. Our minds do this wonderfully. This is useful when we want to problem solve, or create or understand something. However, sometimes that means we can get caught up in negative thought patterns that lead us into experiential avoidance ("my poetry is not as good as Robert Frost's so I should just stop writing it"), rather than allowing us to engage in experience ("I enjoy writing poetry"). Learning to watch our own minds at work can give us enough distance to begin to allow ourselves to break through our experiential avoidance, towards engagement in all kinds of meaningful experiences.
One analysis for the poem has also suggested that we humans are like the fireflies, trying to emulate the stars, but flawed as we are, we can only do it for so long. I think this analysis is a compassionate way of looking at the human condition: sometimes we can achieve 'great things', and sometimes we expose our 'flaws'; sometimes we try to be something that we are not, but return to what we once were; sometimes we switch on and sometimes we switch off; and that's just how it is.