Basically, once you beat the first level in the game, you can summon people into your world by activating a blueeye stone you find on the ground. Once you find your own blueeye stone, you can lay it down and be summoned into other people's worlds to help them. Co-op is the same as singleplayer, and is one way to reduce the difficulty if you're having a hard time beating a level or boss. It's a little more complicated than that once you start considering soul/alive forms, but that's a good basic explanation.
Sounds awesome, I'm hyped for this game now.