Monte Carlo is a technique to calculate the uncertainty in a forecast of future events. It assumes you are using some kind of mathematical model.
Financial planning abounds with such models.
The model is assumed to be a good one. Monte Carlo won't turn a bad model into a good one. Instead of using a single value for each variable in a model, say investment returns, it uses many values. A Monte Carlo engine runs the model over and over again, each time using a different value for each of the variables in the model. Each run is called a trial .