Over the past few decades, scientists have learned that simple rules can give rise to very rich behavior. A good example is chess. Imagine you're an experienced chess player introduced to someone claiming to know the game. You play a few times and realize that although this person knows the rules of chess, he has no idea how to play well. He makes absurd moves, sacrificing his queen for a pawn and losing a rook for no reason at all. He does not truly understand chess: he is ignorant of the high-level principles and heuristics familiar to any knowledgeable player. These principles are collective or emergent properties of chess, features not immediately evident from the rules but arising from interactions among the pieces on the chessboard.
Scientists' current understanding of quantum mechanics is like that of a slow-learning student of chess. We've known the rules for more than 70 years, and we have a few clever moves that work in some special situations, but we're only gradually learning the high-level principles needed to play a skillful overall game.