What do you mean by Filtered indexes

Filtered indexes are a new feature in SQL 2008 and they allow for an index to contain only some of the rows in the table. Prior to this, an index would always have, at the leaf level, the same number of rows as the table. With filtered indexes, an index can be based on just a subset of the rows.When creating a filtered index, a predicate is specified as part of the index creation statement. There are several limitations on this predicate. The comparison cannot reference a computed column, a column that is declared as a user-defined type, a spatial column or a hierarchyid column.A clustered index cannot be filtered as it is the actual table.