In SharePoint, List is like a table with rows and columns.Similar to SQL tables SharePoint List need indexing if contains heavy amount of data else it will degrade the performance of SharePoint application.
So what is indexing?
Indexing in SharePoint list is similar to indexing in SQL database, but in SharePoint list indexing is maintained by SharePoint and not by SQL.
Indexing columns in a SharePoint List will improve the performance. A SharePoint List support maximum 20 indices. And also remember that not all column data types can be indexed.
Now we will move to Query Throttling.
Query throttling is a new feature in SharePoint 2010. It limits the list items that will be accessed in a single request. By default its limit is 5000 for users and 20,000 for SharePoint Admin.
List View Threshold can be overridden by using below property.