A .nomedia file is a blank file placed inside a folder. When the media scanner detects the .nomedia file, the media scanner does not scan that folder so the images in that folder don't appear in the Gallery app. It's a good way of having a folder full of images (like a skin for an app) and not have those images appear in the Gallery app. You can place a .nomedia file in any folder but the filename must begin with a period.
Also, on a Linux system a "dotfile" (a file with a filename that begins with a period) is hidden from standard view, though most file managers have an option to allow viewing dotfiles. So the file is hidden from view and the images in that folder aren't cataloged in the Gallery app.