This is a lock you own.
This is a lock owned by another user.
If there is a lock you do not own on a file you need to use, if you hover you cursor over the lock, it will show you the user that owns the lock.
Contact that user and make sure that they are not currently using that file. After you have confirmed that the file is not in use, you can steal the lock. Right click the file with the lock and select steal lock. The lock will go from this to this .
Once you are done with a file if you will right click it and select Unlock that will remove the lock and make it available for others to use.