While the phrase explicitly refers to the thrush, a case could be made that it also refers to the poem's author, and by extension the entire field of poetry and literature. In the poem, the narrator writes:
An aged thrush, frail, gaunt, and small,
In blast-beruffled plume,
Had chosen thus to fling his soul
Upon the growing gloom.
(Hardy, "The Darkling Thrush," poetryfoundation.org)
The thrush here could represent the writer, who metaphorically "flings his soul" at the feet of the world. The act of writing is of creation, and creation carries with it the risk of rejection. A writer like Hardy can never know if his works will be appreciated or remembered, or if he was simply "flinging his soul" into the gloom of time, where all things will eventually be forgotten. The phrase "frail, gaunt, and small" shows the insignificance of the individual in the larger scheme of things; like the thrush, the writer is just a single person, and regardless of his importance, he cannot hope to have a massive impact except by random chance. However, like the thrush, the writer chooses to make an effort and try to change the world through his creations; the writer cannot lie back and die, but instead is compelled to "sing" in his own unique voice and prose. Through this effort, the writer is able to create change, even if it is small in comparison. Hardy, therefore, is also a "frail, small" creature "flinging his soul" into the world, hoping that it will be recognized and appreciated.