Simultaneous development allows multiple developers to work on the same file at the same time. If you and your coworker are working on different sections of a file, you can both edit the file and then have a program (such as CVS) merge the two versions of the file. Simultaneous development is also useful when you have developers distributed across many time zones, as it eliminates the need to check with other team members before editing a file.
CVS supports simultaneous development without requiring any configuration changes or setup. Simultaneous development is most helpful when you are using text or other line-based files, because CVS merges are based on the changes in lines. CVS is less able to merge binary files.
When multiple developers use the same repository directory, each should check out her own sandbox, develop in that sandbox, commit to the repository frequently, and update frequently. CVS merges most changes automatically, and if you update and commit frequently, there should be few cases where a human has to edit the changes.