It's a podcast with no foul language, no "adult" content etc. In other words, something that is "clean" and appropriate for anyone of any age to listen to. But as far as I know that's something that the podcast-publishers themselves put there so I don't think that in any way is verified by a third party.
From
Apple's iTunes specs:
"<itunes:explicit>
This tag should be used to indicate whether or not your podcast contains explicit material. The three values for this tag are “yes", “no", and “clean".
If you populate this tag with “yes", an “explicit" parental advisory graphic will appear next to your podcast artwork on the iTunes Music Store, and in the Name column in iTunes. If the value is “clean", the parental advisory type is considered Clean, meaning that no explicit language or adult content is included anywhere in the episodes, and a “clean" graphic will appear. If the explicit tag is present and has any other value (e.g. “no") you see no indicator — blank is the default advisory type."