Total available is the amount you have available through the end of the month. Quicken determines this amount from the value of all the inflows and outflows you track in Quicken. Quicken begins with your starting balance for the month, and then adds the amount you saved for the month or subtracts the amount you overspent for the month.
For the current month, Quicken starts this calculation from today and considers all the amounts to the end of the current month. For future months, Quicken starts this calculation from the end of the previous month and considers all the amounts to the end of the specified month.
Quicken can estimate future values based only on the data it has. Inflows and outflows must be entered or scheduled in Quicken to be considered.