El Caganer is a statue, meant to be included with Nativity scenes (which in Catalonia isn't just the Manager, but includes the entire city of Bethlehem, making the birth of the Christ more inclusive and world historical than the modest manger scene), which features a background character of a Catalonian-dressed peasant pulling his pants down and shitting.
I did just say that.
Here. Let me show you.
There are metaphors involved, I guess, but I think fundamentally there's something extremely satisfying about having a statue of a shitting boy at the birth of Jesus Christ. You can talk about the symbolism of the reminder of the earthiness of Jesus's birth as well as His divinity, but really, it's a guy squatting and shitting! How much more do you really need to know?
Apparenlty this is a big deal in Barcelona, where the city council provoked a public outcry by leaving the Caganer out of the nativity scene in 2005. They claimed that a local civility ordinance prevented them from including it (a claim that the head of Parks and Gardens denied). After a "Salvem el caganer" (Save the caganer) campaign, the Caganer returned in 2006.
And there's a stinger. I'm just going to quote from the Wikipedia page on El Caganer here, because there's no way I can top this.
The Caganer is not the only defecating character in the Catalan Christmas tradition—another is the Tió de Nadal, which also makes extensive use of the image of faecal matter (it is a log, i.e. tió, with a face painted on it, which, having been "fed" for several weeks, is told to defecate on Christmas Eve and "magically" produces candy for children, a candy that has supposedly come from its bowels).