The "W" stands for a wash sale. That means you have a loss on the sale, but it is not deductible because you purchased back the same or similar investment within thirty days of the sale. So, the loss is not deductible. You need to make sure your reported sale amount less the cost of the investment equals the wash sale amount entry. They should all be positive figures.
For instance, if you sold the stock for $300 and it cost $500, the wash sale amount would be $200.