"Look on my works, ye Mighty, and despair!"
Basically, this line has two meanings. The literal meaning is "Look at all my great accomplishments, and envy me, because even if you think of yourself as mighty, you’ll never be as great as I am!" The subtle, ironic meaning is: "Look at how all of my great accomplishments have crumbled away into nothingness, and feel sad and hopeless, because even if you’re mighty, the same thing will happen to you after you’ve died!"
The line we’re talking about appears in the poem "Ozymandias," specifically near the end, at the climax (the most exciting part).
Let’s recall what’s going on in that poem. The speaker of the poem reports a story he heard from a "traveller" (line 1). This "traveller" remembers what he saw while visiting a desert in "an antique land" (line 1). It’s the broken and "shattered" (line 4) statue of some king named Ozymandias. The statue’s pedestal bears an inscription, which we read in lines 10–11:
"My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!"
The inscription is telling everyone to look at his "works," whatever they are—maybe glorious buildings, or maybe more statues. But here’s the twist. There’s nothing to look at. There are no "works" in sight, just the "boundless and bare" sands of the desert (line 13). Even the statue itself is a decaying "wreck" (line 13).
As you can see, the words the king says in line 11 are ambiguous: that is, we can interpret them in two different ways, both of which are valid. The poet, Percy Bysshe Shelley, most likely rendered this line ambiguous on purpose. It makes the poem intriguing. It makes the reader think, "Wait—I’m sure the great king, the one who called himself 'king of kings' (line 10) just meant to boast and brag when he talked about his works, and he made sure that the sculptor carved those boastful words into his statue…but now, so much time has gone by that his words have a completely different meaning. A meaning that the king didn’t intend at all! That's ironic."
Even more than ironic, line 11 is chilling. It suggests to readers that, after our inevitable deaths, and after the inevitable passage of time, not only will our accomplishments fade away into oblivion, but even the meaning of our words can warp. We die. Our "works" die. Even our words die. The poem, chiefly through line 11, implies that this bitter truth applies to even the mighty, even a king, even a "king of kings."