"mysterious things" here means things that we could not fully understand when we were children; for example, a child hearing his parents talk could now and then stumble upon something that was still hard for him to grasp, but was a somewhat frequent subject in their conversations. A father could tell his son not to put too much butter on a piece of bread "'cause it clogs up your heart". These little things constituted the "unknown world" in the mind of the child. It was something his little mind was yet to start comprehending.
Student or Learner