Reverse Polish Notation is a mathematical notation which more closely models the way you solve algebraic equations. It uses "Enter" to separate numbers and mathematical operators appear at the end. WRPN uses a stack to store intermediate values which eliminates the need for parentheses to group intermediate calculations.
For example, to add 2 plus 3:
Tradition notation Reverse Polish Notation 2 2 + Enter 3 3 = +
Notice that the two numbers are separated by the Enter button and that the operator (the "+" button) comes at the end.
The elegance of this notation becomes more apparent in the following example (2 + 3) x 4:
You type Shown in display X register Y register 2 2 2 0 Enter 2.0000 2 2 3 3 3 2 + 5.0000 5 0 4 4 4 5 x 20.0000 20 0
The Enter button is only required to separate the numbers (and push the value onto the stack)