What is Index Tuning?

One of the hardest tasks facing database administrators is the selection of appropriate columns for non-clustered indexes. You should consider creating non-clustered indexes on any columns that are frequently referenced in the WHERE clauses of SQL statements. Other good candidates are columns referenced by JOIN and GROUP BY operations.

You may wish to also consider creating non-clustered indexes that cover all of the columns used by certain frequently issued queries. These queries are referred to as “covered queries” and experience excellent performance gains.
Index Tuning is the process of finding appropriate column for non-clustered indexes.
SQL Server provides a wonderful facility known as the Index Tuning Wizard which greatly enhances the index selection process.