Well, usually the players and GM agree on a minimum number of posts to provide. It's a commitment. Usually, it's something like one post a week day, though it may greatly vary.
Most of the time, the game will just flow, just as if some players were more active than others at the game table, while others just sit there saying nothing for a while. It's in combat where that may become an issue, because then each player has to post what their character is doing during the round, and again, and again, until the fight is over. The way it's dealt with depends with groups.
But in the end? It's just a commitment. You commit to post at least once in a while, while you trust others to do the same. Many a time, this commitment ultimately fails. But sometimes, it doesn't.
Oh. And welcome to theRPGsite, rcairns84.