I recently filled up a big table that had an IDENTITY column. IDENTITY columns are "auto-numbering" columns that can be set up to increment every time you insert a new row in a SQL Server table. This can be used to make a convenient ID column.
But if you delete records from a table, or even TRUNCATE the table, the IDENTITY column will not go back to zero, or whatever initial value you set up. To do this, you use a DBCC CHECKIDENT command:
This example sets the IDENTITY column of MyTable back to its original seed value, if that is possible.