can anyone explain ngoptions

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AngularJs

Hi,
can anyone explain this code:
Given this array of items on the $scope:

$scope.items = [{
id: 1,
label: 'aLabel',
subItem: { name: 'aSubItem' }
}, {
id: 2,
label: 'bLabel',
subItem: { name: 'bSubItem' }
}];

<select ng-options="item as item.label for item in items track by item.id" ng-model="selected"></select>

In ng-options we have specified
"item as item.label for item in items track by item.id" can anyone explain this statement