ummm 4.5 is ancient, why did you buy that? It's a set of developer tools for DS 4.5, as of DS 4.7 and up the 4.5 SDK won't work for much. I thought the SDK was part of the distribution of the newer versions of Daz Studio.
SDK = Software Development Kit
It's for C++ development of plugins if you make them. Might still be useful if you program in C++ for Windows or OSX.
Unless you're writing a plugin for Daz Studio, you would have no use for the SDK. It doesn't hurt anything having it around, but without a compiler and some coding skill, it's pretty useless.