This is just one object we could pick out of a whole bunch in the poem, but we think it's a great symbol of feminine appeal, which is such an important theme in this text. The way that ladies dance and drink and dress is an obsession for our speaker, and the poem spends a lot of time on those little details.
Line 19: This is just an earring. Or is it? You see, it happens that this earring, like a lot of things in the poem, is filled with poison! It's gone from being an innocent thing for a lady to wear to a deadly weapon. That's a big theme in this poem: looks can be deceiving. A sweet pretty young lady could actually be a serial killer.