"Kingdom Hearts! Fill me with the power of darkness! [...]"
"You're wrong. I know now, without a doubt. Kingdom Hearts... is light!"
"Light... but why...?"
Kingdom Hearts is one of the major entities in the Kingdom Hearts universe. It is the heart of all worlds, and a source of great power and wisdom. It can be considered an aggregate of hearts. It can be seen as the heart of everything. Due to its great significance, it has repeatedly been sought by the villains of the Kingdom Hearts series, in particular the series's main antagonist, Xehanort.
In the "age of fairy tales", when the World was whole, Kingdom Hearts was believed to be the source of all light. It was protected by its counterpart, the χ-blade. Many came to desire the power of light contained within Kingdom Hearts, and they clashed together, wielding the Keyblades they created in the image of the χ-blade. Their greed and jealousy introduced darkness into the World, and in the aftermath of this Keyblade War, the χ-blade was shattered into 20 pieces (seven of light and thirteen of darkness). Darkness covered the World and Kingdom Hearts disappeared within the darkness.
Since then, certain individuals have been yearning for Kingdom Hearts. Xehanort attempted to forge the χ-blade and summon Kingdom Hearts to restart the Keyblade War, simply because he was curious as to what would happen. Much later, Ansem sought out Kingdom Hearts as he wanted to gain its power and knowledge, while immersing the universe with its darkness. However, Sora opened Kingdom Hearts to find that it contained immense light and used it to defeat Ansem. Then, Xemnas and the Organization XIII came close to creating an artificial Kingdom Hearts that they hoped would restore their hearts, only for it to be mostly destroyed by Ansem the Wise with his specially-built machine, the Kingdom Hearts Encoder. Xemnas fused with his broken Kingdom Hearts, but Sora, Riku, Donald and Goofy managed to destroy what remained of it.
Upon his return, Xehanort again set his sights on obtaining the true Kingdom Hearts for his original plans, this time forging the x-blade by having thirteen darknesses he created to clash with the seven guardians of light and to make the Keyblade War.