Forecasting is about predicting the future value of a measure. There are many mathematical models for forecasting. Tableau uses the model known as exponential smoothing. In exponential smoothing, recent observations are given relatively more weight than older observations. These models capture the evolving trend or seasonality of the data and extrapolate them into the future. The result of a forecast can also become a field in the visualization created.
To turn forecasting on, right-click (control-click on Mac) on the visualization and choose Forecast >Show Forecast, or choose Analysis >Forecast >Show Forecast. When no date dimension is present, you can add a forecast if there is a dimension field in the view that has integer values.