A snapshot view is a copy of the VOB as of the time you last updated the view. When your working directory is in a snapshot view, you work in this private view on your hard drive.
When you check out a file, you check out the version in your snapshot view. Meanwhile, other team members may be making changes to the same file in their own views and checking them in. When you try to check in your changes, someone else may have created a new version, and you must merge the changes in the new version with your changes before you can check in the file.
To minimize the possibility that a new version is created before you check in your changes, update your view before you check out a file to obtain the most current version.