This new feature of the FROM clause allows you to select a sampling of data rows (either a percentage or a number of rows).  Note: the number of rows it returns is completely approximated.

SELECT Field1, Field2 FROM blah TABLESAMPLE (10 ROWS)

SELECT Field1, Field2 FROM blah TABLESAMPLE (20 PERCENT)

This modifier is useful when you just want to see a sampling of the table, but not necessarily the items at the top of the query.

Read the complete post at http://www.dotnettipoftheday.com/blog.aspx?id=182