What is the collections Initializers in .Net

Which allows to initialize a collection also at the time of it’s declaration similar to array which was not possible in the earlier version

Before 3.0 :-

               List<int> li = new List<int>();

               Int[] arr={ 10,20,30,40,50,60};

               li.addRange(arr);

From 3.0 :-

               List<int> li = new List<int> { 10,20,30,40,50,60};