MOOP stands for "Matter Out Of Place". In the real world this refers to anything lying on the playa surface that is not dried mud. It's unnatural stuff that will need to be picked up and removed from the desert— the sooner the better— before it blows away to become someone else's problem.

Jaguar Romano's camp thoughtfully contrasts MOOP Hell (below) with Heaven (above)

At Burning Life, MOOP is any prim that is outside your building parcel, taking up Region resources. MOOP is not our friend. MOOP will be returned to your inventory when found. We will endeavor to give you a heads-up before returning things, but if we miss that part, please don't take it personally! Since cleaning up MOOP is a big time-sink for the volunteer staff, please do your part to keep things tidy by not leaving things around in the first place! "Leave No Trace" and "Don't let it hit the ground!" as they say in the Real World.