Something to know:
Podcast – in common use, the term refers to audio files published to the web along with a news feed, letting users subscribe to a series of shows and download them automatically. Podcasts are a collection of files, such as audio MP3 files or other media files, referenced in an RSS 2.0 news feed as enclosures.
Podcasting – the process of publishing audio files, typically in MP3 format, along with news feeds so that listeners can download them and listen to them on their computer or portable digital audio player.
What makes podcasting special is that it allows individuals to publish (podcast) radioshows, that interested listeners can subscribe to. Before podcasting you could of course record a radio show and put it on your website, but now people can automatically receive new shows, without having to go to a specific site and download it from there.
So, when you see this sign you know that someone is podcasting.