The negative volume lets you subtract one mesh from another.
That way you can, for example, create easily resizable holes directly in PrusaSlicer.
Right-click on a model and pick Negative volume from the context menu. Then select the negative volume shape. You can either pick one of the basic shapes (cube, cylinder, sphere), pick one from the Shapes Gallery or use Load... to import a custom mesh.
The subtraction of the negative volume happens during slicing, the actual model geometry is not modified. That means it is not possible to export an STL with the subtracted negative volumes.