From the introduction:
Welcome to the future, and the next stage of human history, spreading out into the solar system towards the distant stars: welcome to The Expanse. In the 23rd Century, humanity has long since left the cradle of Earth to colonize the solar system. Now an independent Mars vies with old Earth for political power and influence while the settled asteroid belt and the moon systems of Jupiter and Saturn supply the inner planets with the resources they so desperately need. The people of the Belt and the Outer Planets—the Belters—labor and chafe under the rule of the Inners, and unbeknownst to humanity, history is taking a most unexpected turn.
The Expanse Roleplaying Game is a tabletop roleplaying game (or “RPG” for short). It is a game of the imagination, where you and your friends take on the roles of characters in a solar system on the verge of an exciting—and dangerous—new era. It is a moment when people in the right place at the right time willing to do the right thing can potentially change the course of history and the future of humanity, and you can be those people. You may have played other tabletop roleplaying games before, or this may be your introduction to new worlds of storytelling and adventure. Either way, welcome to The Expanse.