What do these lines from Shelley's "Ozymandias" mean? Look on my works, ye mighty, and despair! Nothing beside remains. Round the decay...

It's lines from Shelley's sonnet Ozymandias, and to make sense of it you need to put them in context. Here's the whole poem, with your lines in bold:

I met a traveller from an antique land
Who said: Two vast and trunkless legs of stone
Stand in the desert. Near them on the sand,
Half sunk, a shatter'd visage lies, whose frown
And wrinkled lip and sneer of cold command
Tell that its sculptor well those passions read
Which yet survive, stamp'd on these lifeless things,
The hand that mock'd them and the heart that fed.
And on the pedestal these words appear:
"My name is Ozymandias, king of kings:
Look on my works, ye Mighty, and despair!"
Nothing beside remains: round the decay
Of that colossal wreck, boundless and bare,
The lone and level sands stretch far away.

So, your first line is part of the words engraved on a pedestal, boasting about the great works of the king Ozymandias. But the boast is ironic, because the works have been destroyed by time: only two huge fight and the shattered face of what once must have been a statue remain. "Nothing beside remains", our narrator tells us - there's nothing else left.

And, round the decay of that huge wrecked statue, all there is is sand - metaphorically representing the sands of time, which has brought the mighty (in this sense, quite literally) to its knees.