I'm not sure what the intended effect of the checkbox is, but near as I can tell, it doesn't affect the tag search at all, at least on my end(maybe there's a script that breaks on Firefox). For example, I was just looking at the Furry tag, and no matter whether or not I have the "Show Galleries" checkbox checked, the displayed results are identical.
Now I don't know what the intent behind the checkbox was but I assume it was something like this:
Show Galleries Checked
Visible - Resources
Visible - Pictures uploaded in the Media portion of the site.
Show Galleries Unchecked
Visible - Resources
Hidden - Pictures uploaded in the Media portion of the site.
Now I don't know what the intent behind the checkbox was but I assume it was something like this:
Show Galleries Checked
Visible - Resources
Visible - Pictures uploaded in the Media portion of the site.
Show Galleries Unchecked
Visible - Resources
Hidden - Pictures uploaded in the Media portion of the site.