A transaction is waiting to acquire a key-range lock on an index. This lock has two components:  

• A lock in exclusive mode on the range between two consecutive index entries or rows. Other transactions can't read or modify this locked range.
• A lock in exclusive mode on the index entries or rows. The locked rows can't be read or modified by other transactions.

Key-range locks protect the rows included in a record set, and prevent phantom reads, insertions and deletions when the T-SQL is using serializable transaction level.  

Investigating

• Break long transactions down into shorter ones. See: Managing Long-Running Transactions (TechNet).

• Check the isolation levels for your transactions and update the locking and row versioning behavior if necessary. See: Set Transaction Isolation Level (TechNet).

• Check whether lock escalation is causing blocking problems and resolve if necessary. The Average wait time tells you whether you’re suffering from many short blocks or several long blocks. To troubleshoot blocking, see: How to resolve blocking problems that are caused by lock escalation in SQL Server (Microsoft Support). 

• Check the affected queries and the Top queries table. Tune queries so they run faster and require fewer locks. See: Query tuning (TechNet).

• On the Analysis page, check these metrics for additional details on locking behavior:

• Lock timeouts/sec

• Lock timeouts/sec

• Avg. lock wait time

• On the Analysis page, check these metrics to see whether memory problems or I/O bottlenecks are causing locks to be held for longer than usual:

• Machine: memory used

• Memory pages/sec

• Disk avg. read time

• Disk avg. write time

• Buffer cache hit ratio

• Buffer page life expectancy

See also: Investigating I/O bottlenecks.

• Check the sys.dm_trans_locks DMV for resources associated with lock requests. See: sys.dm_tran_locks (TechNet).

• Consider using partitions to split single lock resource into multiple resources (only available if you’re using more than 16 CPUs). See: http://technet.microsoft.com/en-us/library/ms187504(v=sql.105).aspx.