Yeah the tl;dr is "Germany wins WW1, ect" which comes off a bit Wehrabooish, which is sad because it's way more than that.
The reality is a game that has a trillion times more depth and flavour than the base game, every country having unique paths (ie. actual internal politics rather than HOI4's "I'm Venezuela in the 1940s, I guess"), and a pretty engaging 'story' if you want to call it that. I don't want to spoil too much if you want to jump right in and experience the thrill of looking at America in 1937 and trying to figure out what the hell is going on, but the basic summary is:
Instead of Germany experiencing the massive social upheaval after WW1 it happened in the Entente (because they lost, duh), and in the '20s the UK and France experienced far-left Communist Syndicalist revolutions. The Entente powers ran away to their colonies - the Dominion of Canada and India for the British, and North Africa for France - and plot their return home. In Russia, the Red Army is defeated by the Whites, and President Kerensky is the only thing holding the Republic together and hopefully nothing bad will ever, ever happen to him. Communist Communard France plans to take on Germany and save the world from the chains of capitalism ect, so hopefully Germany doesn't have a financial crisis or something that might make them vulnerable. A whole bunch of other insane stuff happens later, but I'll let that be a surprise.
The main thing setting apart Kaiserreich from the other Alt History wank mods is that it has about 8 years worth of development since it original came from an older Paradox game, so the main draw is less the novelty and more the huge attention to detail.