A slicer is software that takes a 3d model file and "slices" it for 3d printing. It outputs a series of instructions for a 3d printer to build up a physical object, layer by layer (slice by slice). https://en.wikipedia.org/wiki/Slicer_(3D_printing)

Particularly relevant for this discussion is this part of that page, showing the lineage of the various software involved. https://en.wikipedia.org/wiki/Slicer_(3D_printing)#List_of_s...