Locking is a powerful concept in Subversion, allowing single users to gain exclusive access to modifying a file in the repository. Once locked, other users will be unable to commit changes to that file until it has been unlocked. This allows you a strong concept of task and file ownership during your project, aiding division of labour. Even more so, the locking mechanism is very useful in the reduction of conflicts.
To lock a file in SvnX, command-click a file and choose Lock - alternately, if you want to unlock a previously locked file, click Unlock.
Confirm the action by clicking OK.
The file will now be locked for writing by other collaborators until you unlock it (see above) or commit the changes in the file, which will automatically unlock it.