Let me first begin by stating that stories with multiple storylines are not a problem. In fact, if an author does this correctly, then those different plot lines really do add a great deal of depth to a story. Unfortunately, if this is done incorrectly, you end up with stories that become confusing and often very disjointed.
As I read a story with multiple storylines, I try to think about the diagram below. Essentially we have a single storyline running through the book. This is the central story arc represented by the black line.