There are several other theme settings listed on the theme settings page, so have a look at what each of these do. Additional information on Moodle's theme settings can be found at http://docs.moodle.org/en/Theme_settings.
As demonstrated, the Theme list text box is used to control how many and exactly which themes appear in the theme lists for course and user themes. The user themes are the selectable themes in the User profile section when a user edits his or her profile. The course themes are available to administrators and teachers within the course settings page. Both of these themes can be set independently from the main Moodle site theme if allowed by the administrator.
If this text box is left empty, all of the default themes will be available. But, if you wish, you can restrict which themes appear by specifying a comma-separated list of the themes, such as autumn, standard, and orangewhite.