These words are a warning from Ozymandias (which is what the Greeks called Rameses II), generally considered the most powerful of Egypt's pharaohs, to other men of power who happen to see his statue in particular and his other monumental works. He is, in effect, telling them that, no matter how powerful they are, they will never be able to outdo him in magnificence and power.
The irony, of course, is that the poem's narrator tells us that Ozymandias' monumental works are
Two vast and trunkless legs of stone. . . .Half sunk, a shattered visage lies. . . .
In other words, the monument that was supposed to symbolize Ozymandias' greatness and power lies buried in the sand in barely recognizable pieces, "these lifeless things," clearly a symbol not of power but of the decay that comes to all men, even those who were once powerful.
In these lines, then, we are presented with two ironies: first, Ozymandias' greatness is buried in the desert by time and decay; second, those who read the inscription might take it as a warning--totally unintended by Ozymandias--that human power and magnificence are temporary.