It's a really silly, out of character story meant to be ridiculous and crazy (ie-the author was on crack when he/she wrote this...) Usually crack!fics are noted as such by the authors.

For instance: A Buffy AU fic with the Buffy characters re-enacting the tale of Rapunzel with Buffy as the princess and Spike as her dashing prince would be considered a crack!fic. Make sense? :)