Plugins for the Coppermine Photo Gallery are add-ons that use the plugin interface of Coppermine.
Read up the documentation that comes with Coppermine to find out more, e.g. how to install a plugin.

Coppermine version: cpg1.5.x
Plugin name: Monitor Calibration Bar
Plugin version: 1.0
Plugin author: Joachim Müller
Plugin announcement thread: http://forum.coppermine-gallery.net/index.php/topic,58638.0.html

This plugin will display a black-to-white bar on your gallery that visitors can use to roughly calibrate the brightness and contrast of their monitors. Usefull for photography-related sites, where details matter.
This plugin goes unsupported by the plugin author - it is being supplied as a courtesy to end users.

Plugin License: this plugin does not come under GNU GPL, but a slightly different license. It's one that I had to decide for because of the unpleasant experiences with a person who blatantly ripped coppermine, modified the credit tag and offered the package on his site, pretending that he was the one who held the intelectual properties.
That's why this plugin ships under the license that is based on the SMF license. Attached to this archive you should find a file named "license.txt" within the plugin folder that contains the details.