This is to prevent DNewsWeb from displaying too many articles. Related Items brings up a list of articles that are in the same thread as the one you are reading. A thread is a series of articles that have been repetitively replied to. So when you post a new message, that will be the only one in the thread. If someone replies to that message of yours there will then be two messages in the thread.
Some popular threads can get very complicated.
Latest Items reorders the articles so that the most recent to arrive in the newsgroup, is at the top. The dates are not going to be in exact order, as the date time is different in different parts of the world simultaneously.