LOCK ESCALATION:
SQL Server supports Lock Escalation - when the server decides to move from a large number of row or page locks on an object to a table-level lock. It is the process of converting a lot of low level locks into higher level locks.
when a particular query obtains a large number of row level locks/ page level locks, SQL Server decides that instead of creating and granting number of row level/page level locks, it is effective to grant a single table level lock. Or to be precise,
SQL Server upgrades the row/page level locks to table level locks. The above process is termed as lock escalation.