The ratings displayed at the footer of a poem (which only show up after you have rated the poem) are in the format: Rated 8.3 (7.5) by 10 users. The 8.3 here represents the weighted rating of a poem; it reflects the average rating of the poem with the highest and lowest votes dropped and considers only votes cast by active users. The parenthesized number represents this rating if votes by all users were considered. Hovering your mouse over the poem's rating will show the poems individual ratings, both active and inactive.
The rankings on this site are done by true Bayesian estimate, a formula that takes not only rating, but also amount of votes into account. By using this system, a top-ranked poem rated 8.3 by 12 users will not lose it's spot to a poem that one or two people give a perfect ten.