Rather than start with the rules themselves, the story of Rationalist Poker will begin with the objective.
The objective of poker is the same as the objective of everything else: To gain utility. While playing poker, this utility will usually be maximized in the form of money. Money in poker is usually represented by poker chips, which can be exchanged after the game for cold hard cash. Therefore the goal is usually to maximize the net number of chips won, or minimize the number of chips lost. To the extent that you leave with more or less chips than you sat down with, you have won or lost respectively. It is important never to lose sight of this, or you will pay dearly for your mistake.
It is important to note, however, that this is not the only thing poker players value. Most players to some extent value fun, however they see fun. There are those who primarily want to take risk, or pass the time, or hang out with their friends. Often one will be itching for some action, and participate far more than they themselves know they should. Many will value information about what is in your hand, or are seeking to gain experience. A few are experimenting to see what happens. A small number will even be seeking to improve their rationality. A great number will value pride. Some will seek to destroy certain opponents and help others. If you model your opponents as always seeking only to win you will end up with a poor model of their behavior, and that’s before all the biases they have!
There are also poker tournaments, in which the play continues until one person has all the chips and what matters is the order in which players lose all their chips rather than the number of chips you leave with after the game. This determines who finishes in what position, which in turn determines the prizes. Sometimes being the one left with all the chips and thereby winning is most important, sometimes it is not. Here opponents will have even more unique goals. Some will seek volatility, while others will seek survival. Some will want to make sure they win at least some prize while others will only care about winning outright.
There is nothing wrong with sharing many of these motivations, but in a serious game never forget to focus only on what matters: The expected value of your play in terms of chips and dollars.