Kingdom Hearts(and Kingdom Hearts II) is a video game for the PS2 in which you more or less have to save all of existence from the diabolical schemes of the "Heartless" in KH1, and the "Nobodies"(more specifically Organization XIII) in Kingdom Hearts2
But anyway, the main character is the one destined to wield the "Keyblade", which is basically a sword except it looks like a giant key and can unlock any lock in existence, and seal them as well(including the keyholes that link worlds...etc)
So yeah, that basically sums up the basics of what the games themselves are
If you need specific details about any of it(and I do mean any hahaha) just let me know, I'd be more than happy to help