The double exponential smoothing procedure displays the forecasts and their 95% prediction intervals. The prediction interval is a range of likely values of forecasts. Since you do not know the true value of the forecasts, the prediction interval allows you to guess their values based on the time series data. The forecast provides an estimation of the future realizations.

For the sales data, each one of the six prediction intervals tells you that you can be 95% confident that the computer sales in that certain month will be between the shown limits. For example, you can be 95% confident that in the next month you will sell between 324546 and 451030 computers).