"i dont get that part cuz i can do that now lol" This is actually a new feature that allows you to render shadows accurately on a background image, which you can't do now (well, you can using certain material settings which makes the ground plane invisible except where shadows are cast, but it doesn't work well on some kinds of lighting situations). This "shadow catcher" feature is something high end 3D packages have had for a while now (in Max it's called a matte/shadow material), that allow you to composite 3D elements seamlessly into background images, complete with shadows.

---

Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.