Here is the quote you are referring to:
"Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away."
The "colossal wreck" refers to the fallen statue of Ozymandias, the once-great ruler of Egypt who commissioned an image of himself so that the mighty should look on his works and despair. The word "colossal" echoes the word "vast" from line 2, identifying the colossal wreck as the vast stone sculpture. The words "boundless and bare" do not modify the statue, nor do they modify the king's empire. If a comma was placed after "bare," the meaning would be ambiguous, and the words could modify either the "wreck," the preceding noun, or the "lone and level sands," the following phrase. Without the comma, however, the words must be read to modify the "sands." The poetic word order might lead to confusion. To clarify, put the words into the word order normally used in speech, like this: "The lone and level sands stretch far away, boundless and bare." This means that the only thing surrounding the fallen statue is a barren wasteland of desert. The word "bare" means that nothing stands on the sands--no city, no temple, no shrine, no gardens, no fountain--not a single stone testifies to the great kingdom over which Ozymandias once ruled with his "sneer of cold command."