This is poorly phrased, it would probably be better to say something like:It is the idea that the behavior of quantum particles is actually determined by even smaller particles (or something) that we've not yet detected. Probably the most popular example is quantum entanglement.
This is a common misconception or, at best very misleading. It is only true for an unusual notion of effect. (If you pull a shoe out of a shoe box, does noticing it is the right show cause the other shoe in the box to be the left one?)Yet, altering the state of one of the pair can have an effect on the other.