List Threshold and throttling is a new feature in SharePoint 2010 lists. This allow SharePoint administrator to block running heavy queries on SharePoint Lists which has thousands of items. In SharePoint 2007 the max items that can be returned by a list view was about 2000 items and any query over that number can result in low performance and load on the server. SharePoint 2010 lists has been optimized to support millions of items all together but provide SharePoint Administrators control over how many items are being retrieved from the lists at a time using User Interface or Object Model.
See the example below as we have a list with 6000 thousand items. The default threshold limit is 5000 which can be changed in the Web Application Settings.