SQL Server allows you to create, alter or drop indexes online i.e. while underlying table is available to users. Online index operator implements these operations and is a physical operator.
Let us reproduce this scenario with the help of following query.
USE [AdventureWorks2012] ALTER INDEX [PK_Address_AddressID] ON [Person].[Address] REBUILD WITH (ONLINE= ON)
Above plan is self-explanatory, clustered index scan outputs into online index insert operator. Next to that, there is Gather streams operator which only used in parallel plans. This operator consumes several input stream and produce single output by combining all of them.
What happens, if we remove ONLINE = ON option from the ALTER INDEX statement, let’s see.
USE [AdventureWorks2012] ALTER INDEX [PK_Address_AddressID] ON [Person].[Address] REBUILD
Online index insert is replaced with Index Insert as can be seen above and rest of the plan remains same.