Can someone explain to me what bitrate means? Are they of lower quality?
Overall, yes. Lower file size, lower quality. But that doesn’t mean bad quality - if you take care when encoding (which I’m sure Schorman has done), it will still look excellent.
The “full bitrate” versions are equivalent to Blu-Ray releases, and are 30-40GB (like Blu-Rays). The “low bitrate” versions are more equivalent to, say, iTunes/Amazon digital download or Netflix/Hulu streaming.