That's the dominant data structure in the whole program. It's a little different from what's used in APL and J, but essentially it represents an array which carries around its length, shape, and pointer to items. An explanation for the J version is in An Implementation of J [1].

There are a handful of helper functions to get/set various parts of this structure, and most of the functions in the language (and its implementation) take 1 or 2 of these and return 1.

[1]: http://sblom.github.io/openj-core/iojNoun.htm#Arrays