Is my forecast powerful?
The MASE (Mean Absolute Squared Error) is one of the most important indicators for your forecasting model in Tableau.
A value of 1.00 means, that your model is not better than simply taking the last value to forecast (a.k.a. as the naive approach). A value of 0.50 means, that you doubled your prediction accuracy compared to the naive last value approach.
If you want to learn more, Martin has a great course on Udemy called "Forecasting and Time Series Analysis in Tableau and R."