What is lock escalation ?

Answer:

Lock escalation is the process which involves of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks).
Every lock is a memory structure.
More number of locks would mean, more memory being occupied by locks.
To prevent this from happening, SQL Server escalates many fine-grain locks to fewer coarse-grain locks.
Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it is dynamically managed by SQL Server.