The Mood Index is maybe the most important number you can get from the API. It tells you how energetic the crowd is.
It is presented as an integer between 0 and 10, where 0 is the lowest and 10 is the highest.
It's important to understand that the Mood Index is relative to the venue. This means that a Mood Index of 6 at one venue can be very different from a Mood Index of 6 at another venue, but both are just above "halfway" in the Mood scale of the venue. So both venues are just above their respective 'mid-mood' levels.
This is very different from the Loudness Index, which is an absolute measure, and directly comparable between venues.
The Mood Index comes with a descriptive "tag" - a string to describe the Mood Index.