John Keats's sonnet "To the Nile" describes the famous Nile River and considers its place in human history. The sonnet begins with these lines:

Son of the old Moon-mountains African!
Chief of the Pyramid and Crocodile!
We call thee fruitful, and that very while
A desert fills our seeing's inward span:
Nurse of swart nations since the world began,
Art thou so fruitful? or dost thou beguile
Such men to honour thee, who, worn with toil,
Rest for a space 'twixt Cairo and Decan?

In the first eight lines of the sonnet, Keats praises the Nile in apostrophe, a direct poetic address to something that either cannot speak or is not present to respond. The first two lines make it seem like the speaker is reverent and appreciative of the Nile's power and reputation. He recognizes in line 3 that people consider the Nile "fruitful" even though it is in a desert. While the river has been the "Nurse of swart nations since the world began," he asks a rhetorical question: "Art thou so fruitful?" (5-6). This question ponders the accepted idea of the Nile and whether it is true. He then offers another rhetorical question, asking whether the Nile has some magical powers to make tired men think it is more "fruitful" than it is because they desperately need a "Rest ... 'twixt Cairo and Decan."

The line in your question follows in the second part of the sonnet:

O may dark fancies err! They surely do;
'Tis ignorance that makes a barren waste
Of all beyond itself. Thou dost bedew
Green rushes like our rivers, and dost taste
The pleasant sunrise. Green isles hast thou too,
And to the sea as happily dost haste.

Lines 9 and 10 bring up the topic of human folly, or "ignorance." The speaker suggests that "fancies err." This human propensity for error could "make a barren waste / Of all beyond itself." This suggests that humans may be wrong if they are not able to fully appreciate the fertility of the Nile. He follows this statement with descriptions of that fertility: "Green rushes like our rivers ... Green isles hast thou too" (12-13). This repetition of green emphasizes the fecundity of the Nile, while the "barren waste" can only be seen in the flawed imagination of some humans.