A pending, decisive outcome.  An unresolved decision. A choice or effect which could determine success or failure. Imagine a person balancing at the top of a triangle and potentially falling one way or the other, now imagine that one side is a fire and the other side is a safe landing, such is this metaphor. Typically used for major decisions to be made about an endeavor, such as a battle plan during war, success is hanging in the balance of the outcome.(Contrast with After the other shoe drops, Walking a tightrope or Lay your odds.)

“You have to get one more soccer goal for our team to win, the result of the championship is hanging in the balance.”