the LNB in is if you want to hook another single dish to it. the outputs are setup to have one cable from each port going to it's own receiver. what you have is called a DPP (dish pro plus)...and it is very common.
anyways, don't worry about the lnb in port unless you're wanting to hit another bird, like 129, or 61.5 (or many more).
you need to read up in the newbies section and learn a bit and post tons of questions.
there's tons of knowledgeable people here to meet and will give good advice.