Cash Days shows how long it takes for money to go through your business, from sales to getting paid. It's calculated by adding your aged receivable days (the average of how long it takes for your customers to pay you) to your work in progress days (the average of how long it takes for you to raise sales after completing work - if you're a service business) and adding to your inventory days (the average of how long it takes for to sell of stock/inventory after it's purchased) minus your aged payable days (the average of how long it takes you to pay your suppliers) for the current period.
If you can see a negative number in your clients account, this is a good thing. Negative cash days are good because it means your client is getting money in faster than they're spending. Negative cash days means the accounts payable is greater than the accounts receivable.
If you select a finalised month (see red square above) that isn't the financial year end then the figures will be annualised for the current financial period.
The little coloured number in the lefthand corner is the difference in the cash days between now and the previous financial year.