The Great Wars submod is misnamed, really-- it was named that before this mechanic was added into vanilla. What it does currently is make it so that once Mass Politics is invented (so sometime after 1900) countries can enter into a state of "total war" whenever two GP's are at war. When that happens, they will call all allies and also seek out enemies of their opponent to ally with and call into the war. They also get a new Dismantle Empire wargoal, which if enforced will dismantle the opponents territory and split up all colonies among the victors. This wargoal is occasionally handed out as a CB to countries that are very hostile to each other.
That's it, in a nutshell.