Essentially it gives you loads of kit templates, collars and kit manufacturers to work with to get the actual kit design (there's also an advanced mode to combine kit templates, but I don't use it much). You can add in the team badge, sponsor and even a kit manufacturer of your own if the one you want isn't there, as well as allowing you to align and size them however you see fit. There are also four different lighting modes: my first few posts in here were done with mode A, but getting colours right is so much easier with mode D.
Overall, it's very simple to use and really streamlines the whole process of kit-making (hence how I can keep making new ones fairly quickly), but you are limited by what templates are already there. Of course, you can make your own (like I had to for that Penistone Church away kit) but this can be tedious depending on what image-editing software you use. I'd definitely recommend SSD though; if not for it, I wouldn't have been able to get into kit-making myself, given that I can't afford Photoshop right now or anytime soon.