Mastering is an illusive process to most artists. I think of it like this; mixing is an art and mastering is a science. It's a process that makes the song sound consistent across all playback systems, translates the files to right digital formats for distribution and adds a gentle layer of sparkle. When your song appears in a shuffle with all your favourite tracks, it will meet in volume and tonal fidelity. When mastering is done poorly it will degrade all of the hard work that has gone into the track up until that point. So special attention is taken to ensure every frequency of the tune is exactly where it needs to be. And the song is presented in it's best form.
The artist is delivered a set of digital files which will be ready for any distribution including streaming services, video collaboration or Triple J Unearthed.