Budget Amount is your Budget for the Year (see below for changing)
Encumbered is the amount held for Books ordered but not yet received.
(ie money at this moment is not available to spend as it is accounted for when ordered).
Amount Expended is the cost of Books actually received.
Balance Available is the total balance available to spend.
(although you should account for Books in the Encumbered account you may not receive)
When you are 30 days or less before the Budget Year end this warning message will be displayed
The Budget Year End and the Budget Amount is set under Maintenance.