(aka "Counting your chickens before they hatch")
Sometimes, managers will make commitments to their boss based on uncertain outcomes--this is the nature of forecasting. In some cases, however, such estimates will make overly-optimistic predictions about the future -- depending on an outcome (a project, etc.) with a low chance of success. In many cases, the manager's subordinates are then expected to expend extra (and often heroic) effort in order to ensure that the project/whatever does succeed.
Like the Aesop fable, this is counting the chickens before they hatch. And like in Aesop's fable, it's not a good idea.