Please explain this IIFE?

Please explain this IIFE?

hi

Have a look at this code :

<script>
(function ( $ ) {
var shade = "#556b2f";
$.fn.greenify = function() {
this.css( "color", shade );
return this;
};
}( jQuery )); 
</script>
<body>
<span id ="id1">Enter Your name</span> <br />
<p id = "x">BYE</a> 
</body>

IIFE return in red does not have any effect on any element in <body> tag.So what is the use of IIFE here? Can't we make "this function" instead of IIFE?
What exactly "this" in IIFE refers to?


Waiting for your response!!!!!


Regards 
Varun