Seriously what is it
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.
If I stuck it in my folder called notifications would it hide those files in it from the music gallery
I'm not sure, only one way to find out You can always delete the .nomedia file if it doesn't work.
Where can I generate a nomedia file
A .nomedia file is just a blank text file but has the filename of ".nomedia", any text editor can create a new file but make sure you save it with the .nomedia filename.
fwiw, ES FileExplorer has the option to easily create a new blank file in any directory.
Related question... is there a reference page that lists "period file" names for use in Linux ...my search syntax for this is not getting any results.
Thanks in advance,
And here's the reason behind the .nomedia files:
It was Google Play's compliance policy (not sure if still is) that no internal app files should be unintentionally recognized by any of the media players/galleries. Hence the file, cheers.
What happens if I delete one?
Nothing bad. If it's in a folder with media files - your media browsers/players will start to read those files.
Separate names with a comma.