This is the most common operation when using a version control system. When you checkout, your working copy contains a bunch of files from the repository. You modify those files, expecting to make your changes a part of the repository.
With most version control tools, the edit operation doesn’t actually involve the VCS directly. You simply edit the file using your favorite text editor or development environment and the VCS will notice the change and make the modified file part of the pending changeset.
On the other hand, some version control tools want you to be more explicit. Such tools usually set the filesystem read-only bit on all files in the working copy. Later, when you notify the VCS that you want to modify a file, it will make the working copy of that file writable.