Unexplainer shrinkage (Markdowns) refers to the consumption of goods whose origin is not clearly identified.
It corresponds to a discrepancy between theoretical inventory (what should be available according to orders and sales) and actual inventory (what is actually present).
For example, I have recorded actual consumption of 120 burgers, I have sold 100 of them in my burgers, I have booked 15 extra burgers as a loss (fridge not closed properly), so I have 5 burgers whose absence I can't explain. It could be a loss that hasn't been entered, or it could be overconsumption in the recipes (the recipe calls for 1 ground steak per burger, but the cook uses 2).