The first vital point to note is that the poem is a sonnet in a traditional 14 line, 8-6, set-up with iambic pentameter.
Written in an "antique land" shows that the author was attempting to distance himself from the so-called king indicating the faded view of the past king Ozymandias.
Framing the sonnet as a story we are not seeing the statue with our own eyes, so to speak, we hear about it from someone who heard about it from someone who has seen it.
The speaker recalls having met a traveller "from an antique land," who told him a story about the ruins of a statue in the desert of his native country. Two vast legs of stone stand without a body, and near them a massive, crumbling stone head lies "half sunk" in the sand.
Shelley's description of the statue works to gradually build up the figure of the "king of kings":
First we see only the "shattered visage," then the face itself, with its "frown / and wrinkled lip and sneer of cold command". Then we are introduced to the figure of the sculptor and we are able to imagine the living man sculpting the living king. Then we are introduced to the king's people in the line; "the hand that mock'd them and the heart that fed."
The traveller told the speaker that the frown and "sneer of cold command" on the statue's face indicate that the sculptor understood the passions of the king. A man who sneered with contempt on those weaker than himself, yet he fed his people because of something in his heart ("The hand that mocked them and the heart that fed").
The double meaning of "mock'd" should also be pointed out here. This verb originally meant "to create/fashion an imitation of reality" (as in "a mock-up"), as well as "to imitate" (as in "mock velvet"), before meaning "to ridicule" (especially by mimicking). In Shelley's day the ridicule meaning was the main meaning, but in this poem "the hand that mock'd them", we can read both "the hand that crafted them" and "the hand that ridiculed...