They are, stated plainly, defecation. Shitting. Pooping. Dropping trou. Laying down wolfbait. (Numerous other colorful euphemisms).
"The Dropz" can refer to either the shit itself or any area that is used for shitting, be it a pit or a toilet.
All Warhammer greenskins place importance on relieving oneself. In Warhammer 40000 Orks are civilized enough to use outhouses and toilet paper, in Warhammer Fantasy the greenskins simply prefer to shit in the dwelling spaces of other peoples.
In both settings the "result" of Dropz are building materials. Fantasy greenskins sculpt effigies of their gods, mainly Gork and Mork (or was it Mork and Gork?) out of it. 40k greenskins prefer it as a binding agent.