The actionscript code it put in the button instance. The "on (relaease)" commands the following code to be execusted when a button reaches its "release" state.

on(release){

This statement sets the value of the dynamic text field "output" equal to the value entered in to the text boxe "inputa" plus "inputb".

output = Number(inputa) + Number(inputb);

Number() evaluates and returns the numerical value of any input in the text field making text entries null entries.