"La Belle Dame Sans Merci" is divided into twelve four-line stanzas, called quatrains. Each of those quatrains rhymes according to an ABCB pattern. For example, take a look at the first stanza: the second line rhymes with the fourth: "loitering" and "sing."
That covers the rhyme, but what about the meter, or the pattern of stressed syllables? The basic meter of the poem is iambic tetrameter. Before you fall asleep at your computer, let us explain: "Iambic" refers to the pattern of unstressed and stressed syllables. One iamb is an unstressed, followed by a stressed syllable: da-DUM. "Tetrameter" tells you how many iambs you'll find per line. "Tetra" means four – so there are four iambs per line. Iambic tetrameter. Check it out in line 1:
"O what / can ail / thee, knight / at arms,
We've put the syllables that you would stress more as you read it in bold face, and we've divided up the four iambs.
But something strange happens in the fourth line of each quatrain. There are only three stressed syllables in the fourth line of each quatrain. This isn't a mistake on Keats's part. The fourth line is consistently shorter. Even if you're not used to counting stressed and unstressed syllables, you can tell just from looking at the page that the fourth line is always shorter. What's the effect of this shift in the rhythm? It's an open question. Feel free to come up with your own answers.
Furthermore, "La Belle Dame Sans Merci" is a ballad, which is an old-fashioned, folksy style of poem that typically tells a story. Ballads use simple language that would appeal to less educated people, like farmers and laborers. Ballads were primarily an oral form – people would memorize them and pass them on to their friends and family by memory, rather than from a book. Poets like Keats tried to mimic this style in their written works. Many of the Romantic poets liked the deceptively simple form of the ballad. William Wordsworth and Samuel Taylor Coleridge famously kicked off their careers (and arguably the whole Romantic literary movement) with their collection of poems called Lyrical Ballads.