This system is remarkably simple. Whenever one member of a team needs to modify a particular file, it is locked, so that no one else can make changes to it until it is unlocked again.
Think of it as setting a file as read only for everyone else while you modify it to your pleasure; once you are done, you can "unlock" the file again and other people can begin their work.
The disadvantages here are quite evident. Firstly, you are at the mercy of the first person who locked the file, even though in most cases the changes you might make would not conflict with what they are doing. This is clearly a loss of productivity, especially if that file is essential to your work.
Then again, if a person locks a file and forgets to unlock before going on a vacation / quitting his job / or even going to sleep, you are at a deadlock until an administrator frees the shackles.
Finally, it doesn't even really solve the problem completely since changes in one file may still conflict with the changes being made in another.