What is mean this words in as3

Question

i am new to as3 please help me about this words and how can i use it
_root
Math.atan2
onEnterFrame
Math.sqrt
and the word " this " in as3
*and what is the difference between this words ?
_x and x
_xmouse and mouseX
_rotation and rotation

Answer 1

_root (as2) = MovieClip(root)  (as3)
Math.atan2 (as2) = Math.atan2 (as3)
onEnterFrame (as2) = addEventListener(Event.ENTER_FRAME,somefunction);
Math.sqrt (as2) = Math.sqrt (as3)
this (as2) = this (as3)
_x (as2) =  x (as3)
_xmouse (as2) =  mouseX (as3)
_rotation (as2) =  rotation (as3)