What is Reverse Polish Notation?

Reverse Polish Notation (or RPN) is simply a different mathematical way of laying out equations, take for example the expression:
7+ 9
The layout is <First number> <Operator> <Second Number>, in computing we usually refer to the numbers as 'Operands' - another name for this layout is 'infix'. RPN lays out expressions with the Operands first and the Operator later. So our expression becomes:
7 9 +