I am a starter to javascript. I know to write JS userdefined functions. But recently I came across some thing that I can’t recognize. Can anyone explain to me what this is?

(function( window, undefined ) {

    var jQuery = (function() {
    });
           window.jQuery = window.$ = jQuery;
})(window);

What is the meaning of this? When I Google javascript functions I am getting only

function foo(){ 
    alert("This is an alert"); 
}

I know to use these type of functions