CBR -- Constant Bitrate, which is exactly what it says; the bitrate that OBS streams at will mostly hover around the number specified.
Quality 10 -- It's just the maximum quality you can assign to VBR (Variable Bitrate); the Twitch.TV staff prefers people streaming at CBR actually. Which is why I recommend it to most people. VBR has the potential to look (slightly) better but it also has the potential to cause traffic spikes for yourself, for Twitch and for the viewers. What VBR will do is use little bitrate if the scene is somewhat static and it will spike in bitrate whenever the scene becomes more active or complex.
In my case you could call it an experiment; as I said, I've tried numerous different combination of settings in an attempt to get Minecraft to look better and these somewhat specific settings I arrived at (it's not just the VBR, it's everything, including the custom buffer size, the x264 custom parameters) seem to have peaked its streaming quality for me.
Although I probably will end up going back to CBR, since it's recommended by Twitch. And I'd rather not be another reason why its service has dropped sharply over these past few months -- You see, it is a lot easier to load balance CBR streams than it is to load balance VBR streams. Even if the CBR streams have a constant throughput that is higher, it is mostly constant. So can easily be balanced out across the entire infrastructure. VBR streams are unpredictable, erratic. Which makes it far harder for the system to deal with. Which is part of why Twitch has been performing less than stellar lately.
Quick question, long-winded answer... :P