AngularJS: What compile function does

Question

i am new in angular js.

just like to know about few line like what they does.

app.directive('fancybox', function($compile) {
return {
restrict: 'A',
replace: false,
link: function($scope, element, attrs) {

$scope.open_fancybox = function() {

var el = angular.element(element.html()),

compiled = $compile(el);

$.fancybox.open(el);

compiled($scope);

};
}
};
});

below these lines are not clear.var el = angular.element(element.html()),  

compiled = $compile(el);
$.fancybox.open(el);
compiled($scope);

1) what $compile does and what it return after compile ? 2) when we need to compile scope like compiled($scope);

i have test the if i comment this line compiled($scope); then value is not putting into expression.......why ?

please guide me in detail. thanks