Stockout = days when the product is out of stock.
To calculate the forecast, we remove stockouts. The forecast is largely based on the sales volume, which is sales/days in stock.
You can change settings so that stockouts aren't removed for purposes of calculating the forecast (found in Account > Settings > Forecast - then uncheck the box for Adjust Forecast using Out of Stock Information. This should be done if you allow customers to order even when a product is out of stock. You can also set this on the SKU level instead of the store level if needed.