"O WHAT can ail thee, knight-at-arms, Alone and palely loitering? The sedge has wither'd from the lake, And no birds sing.
The poem opens with a question: an unnamed speaker asks a "knight at arms" what's wrong, or what's "ail[ing]" him.
Something is clearly wrong with the knight – he's "loitering" by himself around the edge of a lake, and he's "pale."
The speaker says that the "sedge," or marsh plants, have all died out from around the lake, and "no birds sing." So we're guessing that it's autumn or even early winter since all the birds have migrated, and the plants have "withered."
The presence of the "knight at arms" reminds us of medieval fairy tales with knights and ladies in towers. We think that this is the response Keats intended
Stanza 2, Lines 5-8
"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.
The first part of the stanza echoes the first line of the poem word-for-word. Apparently the knight doesn't answer immediately, so the unnamed speaker has to repeat the question.
This time, we get two more adjectives to describe the knight: he's "haggard," or worn-out and tired-looking, and "woe-begone." The knight is obviously both sick and depressed.
The last two lines of the stanza do more to set the scene: the squirrels have finished filling up their "granary," or storage of food for the winter, and the crops have already been harvested.