This is a great feature as I know many of us remember creating files and sharing with others only to find out down the line that someone edited the file to an extent that made it unusable. Then you would need to go into version history, etc. I remember a ton of conversations with people being upset about others editing their files.
So, how can you lock a file, it is really quite simple.
Choose right-click on a file in Drive, choose "File Information", and choose "Lock". Of course you can do the same to unlock. You can also unlock the file by choosing unlock in the file itself.
Embedded below is a 1 minute and 6 second YouTube tutorial.