This means:
* The first and third lines of each stanza rhyme (A and B).
* The second and fourth lines of each stanza rhyme (C and D).
* This pattern continues throughout the poem.
Here's an example from the poem:
> O what can ail thee, knight-at-arms,
> So haggard and so woe-begone?
> The squirrel's granary is full,
> And the harvest's done.
Notice how "arms" rhymes with "done," and "begone" rhymes with "granary". This pattern is repeated throughout the poem.