Incidentally, 'his quotation' is a bit inaccurate (in a way that bothers me rather more than it should!); it's a line frequently quoted from one of his works. Besides,
si monumentum requiris circumspice to quote the memorial to
Christopher Wren - Wikipedia, the free encyclopedia set in the floor of St Paul's Cathedral: "if you're looking for a monument, look around you". OK, in that case the monument is a real concrete thing; but when Horace (?) wrote
exegi monumentum, he was referring to a body of literary work. I think the same applies to Keats.
So while I applaud Anglika's answer for its clarity and conciseness, I think there's no substitute for reading the words in their context to find out what an author 'meant'.