I can try, but I want to know how much you need to know (so I know what to put in spoiler boxes).
But to start out simple, what is kingdom hearts? Well first you need to know that there's more than one kingdom hearts. There's kingdom hearts as a gathering of hearts, the kingdom hearts summoned by the clash of light and darkness, and the door (well the door leads to kingdom hearts, but details).
But most importantly, kingdom hearts is light.