If you don't want to encourage concurrent development on files belonging to a file type then the 'Automatically Lock when Checking Out' flag should be set as described in the section called “File Type Flags”. When a user edits a file the file will be automatically locked. When another user attempts to edit the file he/she will be warned that another user has locked the file. Other users will not be able to submit an edit to the file until the lock has been released.
This is useful for file types where merging is not supported. An example might be image files, which would belong to the 'image/generic' file type.