The significance of the end game in the novel is that Ender discovers, after it is over, that it was all real. The entire time that he had been doing the "simulated" interactive computer games, they weren't actually "simulated"; rather, he was fighting real buggers, in real battles, in real situations. The increased intensity of his battles in the zero gravity room were all just preparation for the computer games, where he was given "hypothetical" situations that, with his crew, he strategized to defeat. And these battles increase in intensity, relentlessly, pushing Ender and his team to the ultimate limits. The entire time they were sitting in little rooms, with controls and computer screens in front of them, they were in reality, controlling real ships, real captains and battles, and real battle strategy.
The end game that they participate in is the final battle between humans and the alien buggers in the frontiers of space. Ender takes everything that he learned from his rigorous training and practice exercises, and gives it his all to win what he thought was a computer simulated battle. When he is finished, he is told that what he had done was not fake, but real; this knowledge sends Ender on a search for meaning and redemption. Despite the fact that he supposedly saved the human race and became an immediate hero, he still feels a huge sense of betrayal, guilt and ambivalence about the role that he played in it.
I hope that those thoughts helped a bit; good luck!