When you create an index in the database, the information used by queries is stored in index pages. The sequential index pages are chained together by pointers from one page to the next. When changes are made to the data that affect the index, the information in the index can become scattered in the database.
Rebuilding an index recognizes the storage of the index data to remove fragmentation. This can improve disk performance by reducing the number of pages reads to obtain the requested data.
DBCC INDEXDEFRAG - Defragments clustered and secondary indexes of the specified table or view.
