If there’s more than one person working on a project, chances are (although slim) that at some point two developers work on the same piece of code and check it in. To clarify, let me give you an example.
The repository is currently at revision 5 and contains a file named ‘README’. Revision 5 of that file contains a single line: ‘This is a README file’.
Now, both you and your colleague check out r5 and edit README.