Forecast is a great option when you have Date fields in your Data Source. You can only add a Forecast when you're displaying a Measure through a continuous date. Tableau calculates the forecast based on the existing data points. To add a Forecast, drag and drop the option in the View or select Show Forecast among the right-click options.
When you add a Forecast, the Measure is replaced by a Forecast Measure, and Forecast Indicator is added in Color. Here's an example of the Forecast of the Profit by Continuous Quarter of Order Date:
When you right-click on the View, if you hover over Forecast, you can find a description of the Forecast ...
