In a multi-access system it may happen that two users want to alter the same record at the same time. 

If User A changes the data in a record and saves it back shortly before User B does the same then User A's changes will be lost.

To avoid this, the operating system would use a system of record locking. When a user opens a record on a file it is flagged as 'locked' and will prevent another user changing it. When the record is saved back to the file it is unlocked.

The operating system on a PC usually locks a file when it is opened - or makes it read-only. This means that no two programs can change the same file at the same time.