The operand is decremented by one.
A pre-fixing decrementor will subtract 1 from the operand value before it is used in the expression.
A post-fixing decrementor will subtract 1 from the operand after it is used in the expression.
Be careful how you use this pre/post placement as you can easily generate 'off by one' errors in your algorithms by placing the operator on the wrong side of the operand.
This operator is more or less functionally equivalent to:
anOperand -= 1
which is equivalent to:
anOperand = anOperand - 1