The Prologue is an excerpt from the Encyclopedia Galactica. Think the Encyclopedia Britannica, only in the future of space.
Like Britannica, the only future people who actually own the Encyclopedia Galactica will be college professors and kids who receive the complete set as a Christmas present from their grandfathers (and then never read it).
This particular entry is about a man named Hari Seldon, who is one smart mathematical cookie. Seems he was the Albert Einstein of a discipline called psychohistory. His story, as told by one Gaal Dornick, will be coming in the tale to follow.
Warning! Before proceeding, let's get this one teeny-tiny factoid out of the way: psychohistory is an actual field of study. Its goal is to use the methods of psychotherapy as a means to look at historical cultures (hence the name). It remains a very controversial field of study on the battlegrounds of academia.
More important to us though, it's extremely different than the psychohistory Asimov imagines in Foundation. Just remember to add the word "Asimov"to your web searches, and you'll be fine.