In the below HTML code I have a onchange="myFunction(event)" and I have a function called myFunction

My first question is what does the word event mean in the function call and my second question is what is the argument that I need in the function argument list.

Code:
<html>
<body>

<input type="text" name="MyTextbox" size="40" value="" maxlength="160" onchange="myFunction(event)">

<script language=javascript>
function myFunction(what_goes_here?)
{
  //
  // how to process the argument?
  //
}
</script>

</body>
</html>