When someone, most likely a prepubescent 13 year old, writes a sex definition for something which has nothing to do with the word you looked up.

Theoretical example - I look up forum troll, because i've seen it used extensively on forums, and never understood what it meant. One of the definitions will most likely be something like this -

"While ur fucking ur girl from behind in teh dark, u put a dildo instead of ur dick in her hole, u then grab a cardboard box full of troll dolls and begin throwing them at her, all the while yelling ATTACK OF TEH FORUM TROLLS!!!"

A typical urbandictionary definition deserves deletion, it contributes nothing to the viewer's knowledge, and only serves to prove how idiotic the poster is.