What is the meaning of components inside array when declaring angular js module?

I am new in angular js. I have the following code

angular.module('app', ['components'])

.controller('MyApp', function($scope) {
  $scope.message = "Hello World!";
});

what is the meaning of components here ?

I tried to find something on angular js documentation https://docs.angularjs.org/api/ng/function/angular.module

but there when they declare a new module the array is empty, in my case the array is not empty - so what does it means ?