Welcome to Playrite!
It's fairly simple, you write your chapters and others contribute by either 'add' to continue your story or 'fork' to branch off.
The most voted up chapters will become the default story line.
Each chapter by default can only be edited by its original author, unless author granted permission to friends or 'ritepal', in Playrite term.
Ritepal can be granted by author to any of author's followers.
Swipe left/right to browse all forks and vote up the one you like.
Have fun!