the purpose of the lock object is to prevent two transaction
changing the same data at the same time that is for the lock
object.
           there are three types
          
           1 exclive lock
           2 shared lock
           3 ecclive but not cumulative  

Lock object is a virtual link of several SAP tables which 
is used to synchronize simultaneous access by two users to 
the same set of data (SAP lock concept).
              [or]
    when two users simultaneously attempt to access the 
same data record, this is cynchronised by a lock mechanism.


To set the locks, 

you must define a lock object in ABAP dictionary. The name 
of lock object should start with 'E'.
Thd Fun Modules for requesting and releasing lock which are 
create automatically when the lock object is activated must 
be linked to the programming of the relevant online 
transactions.

They are 3 types:

 1) write (exclusive lock)
 2) read  ( shared lock )
 3) exclusive but not cummulative

Function modules :
 1)ENQUEUE_FSFLIGHT  (to lock the object)
 2)DEQUEUE_FSFLIGHT  (to unlock the object)