First of all, the worlds of Myst and Riven were "created" or "described" (this is a debate within the D'Ni culture) by a civilization known as the D'Ni. This is a race of people that has learned how to write special kinds of books using a certain fabrication process for paper and ink. It turns out that when they write a book that describes a world using this secret process, then the book becomes a portal to that world.
What's more is that the book can be edited, supplemented, rewritten or scrapped all together! As the book's contents change, so does the world it describes. In fact, if a world is not "designed" or "written" well, then the logical contradictions of that world will not allow it to continue to exist. Hence, some D'Ni worlds are "unstable", like Riven, when written by someone who (inadvertently or not) includes flaws in the book.
As described in the fictional books, there is a philosophical conflict within the world of D'Ni. The crux of the matter is this: When a book is written, is the D'Ni author who wrote the book like a god of some kind who created the world? Or do we exist in a universe of infinite possibilities, in which case a book that is written doesn't create a world, it just opens up a doorway to one of the infinite universes you have by chance described in your book?
If you play Myst, you will get some flavor for this notion. By reading the journals of Atrus, you'll get some idea of what this process entails. First, you have to write a book and "activate it" (with some kind of mystical or perhaps mechanical power source). That enables you to link to the world you've written. Of course, you had better take a book with you that describes your own world or you'll never be able to get back! A common D'Ni practice is to sketch the outlines of a world, then visit and revisit it frequently as the writing is "refined", testing and experimenting in a scientific way to see what works and what doesn't. Of course, if the world has sentient inhabitants, there are ethical issues involved with tampering with those people, issues on which there is sharp disagreement among the D'Ni people, as you might imagine.
In the long history of the D'Ni, the events described in the games "Myst" and "Riven" are at perhaps the end of a long cycle. The D'Ni culture is all but wiped out thanks to events described in the fictional books. Only Atrus, Catherine, Gehn and a few others remain, and they battle over the D'Ni legacy. If one faction or the other should prove victorious, then that could fundamentally alter the role the D'Ni play in the future. In "Myst", your job is to help Atrus out of a tight spot caused in large part by his two sons, Sirrus and Achernar, but also by his father Gehn, who has taken Catherine prisoner in the world of Riven.
At the end of Myst, Atrus asks you to come back later when the book he is working on (the description of the world of Riven, as it turns out) is more complete, or more stable, and thus safer to travel around in. Thus, when Riven opens, your apparent goal is to help Atrus rescue Catherine and somehow stop Gehn from interfering maliciously in the world of Riven. Of course, as events unfold in Riven, you may find your point of view and your goal changing depending on the circumstances, but at least you know the background, which may help explain some of the things you see and hear in the game.
So, should you play Myst first before you play Riven? Sure. They're both great games
