I dont get what this event:Event thing is

I dont get what this event:Event argument thingy is in this code plz explain to me in a simple explanation im a total begginer

"import flash.events.Event;
//First I defined a function where all of
//the code needed to start the game is placed
//This includes listeners, variable definitions, and other stuff
function beginCode():void{
//Adds a listener to the paddle which
//runs a function every time a frame passes
mcPaddle.addEventListener(Event.ENTER_FRAME, movePaddle)
}

function movePaddle(event:Event):void{
//The paddle follows the mouse
mcPaddle.x = mouseX - mcPaddle.width/2
//If the mouse goes off too far to the left
}

beginCode();
"