What is the meaning of "boundless and bare" in "Ozymandias"?

Shelley is referring here to the vast expanses of desert that surround Ozymandias's crumbling wreck of a statue. The desert is boundless in that, as far as the eye can see, it never ends. Whichever way you look, in whatever direction, all you can see is desert, desert, and even more desert. The desert is bare in that there are no natural features to be seen: no trees, no rocks, no plants or animals.
What Shelley is doing in these lines is emphasizing the arrogance of this long dead Pharaoh. He assumed that this statue would stand forever, a living embodiment of a great man and his earthly achievements. But the statue is no more. It has long since crumbled into the sand. Nature, in the form of the bare, boundless desert, has asserted its superiority over man, reclaiming the land that was once occupied by this statue.
Like the good Romantic he is, Shelley believes that man is a part of nature, and as such should not stand apart from it. He should recognize that he is intimately at one with every living thing, held together with rocks and trees, mountains, plants, and animals in a vast, all-encompassing unity. Ozymandias failed to realize this; he arrogantly asserted himself over nature and the rest of humankind. And yet now he's nothing more than a forgotten Pharaoh with a crumbling statue.
https://www.poetryfoundation.org/poems/46565/ozymandias