OGG is an open-source audio container, most commonly carrying Vorbis-encoded audio (similar quality to MP3 but patent-free at the time of its creation). Used heavily in open-source software, video games (Spotify shipped OGG Vorbis for years), and Linux audio. Less universal than MP3, older iPods and some legacy hardware don't read it.
How to open
VLC, foobar2000, every modern web browser (HTML5 audio), most Android players, Audacity. Apple devices typically need a third-party app.