On a literal level, this poem is about a hawk roosting in a tree and reflecting on its body, its predatory powers, and its superiority to other creatures ("I kill where I please because it is all mine").
On a metaphorical level, there are several "messages" or themes that one could take away from this poem. Arguably, the main message here is that we can view the hawk as a symbol for humanity's arrogance, tyranny, and obsession with power and destruction. Thus, the hawk in this poem is like a mirror, reflecting back some of our own less-than-desirable human traits.
The hawk is rather conceited and believes nature has been designed solely for him. Hughes writes, "The air's buoyancy and the sun's ray / Are of advantage to me; / And the earth's face upward for my inspection." According to the hawk, everything in nature was designed to serve him, and this sentiment speaks to humanity's beliefs regarding its superiority to nature and to other beings.
The poem's final stanza reads, "Nothing has changed since I began. / My eye has permitted no change. I am going to keep things like this." The hawk believes he will always be superior and powerful, that he is in control of whether or not things change, and even perhaps that he is capable of living forever. This speaks to humanity's obsession with cheating both death and the natural cycles of the earth. Just as humans are arrogant and believe themselves to be indestructible and immune to nature and its changes, the hawk also believes itself to be all-powerful.