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 of the tangent of "inputa".

output = Math.tan (Number(inputa));

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