The SQL Server Database Engine automatically modifies indexes whenever insert, update, or delete operations are made to the underlying data. Over time these modifications can cause the information in the index to become scattered in the database (fragmented). Fragmentation exists when indexes have pages in which the logical ordering, based on the key value, does not match the physical ordering inside the data file. Heavily fragmented indexes can degrade query performance and cause your application to respond slowly, especially scan operations.
You can remedy index fragmentation by reorganizing or rebuilding an index.
Reorganizing an index uses minimal system resources and is an online operation. This means long-term blocking table locks are not held and queries or updates to the underlying table can continue during the
ALTER INDEX REORGANIZE transaction.
Rebuilding an index drops and re-creates the index. Depending on the type of index and Database Engine version, this can be done online or offline.