Did you come from the Nock 4K thing seen in https://urbit.org/docs/tutorials/nock/definition/ but without the rest of the stuff on that page? It’s explained in words there:

> / (slot) is a tree addressing operator. The root of the tree is 1; the left child of any node n is 2n; the right child is 2n+1. /[x y] is the subtree of y at address x.

> For instance, /[1 [531 25 99]] is [531 25 99]; /[2 [531 25 99]] is 531; /[3 [531 25 99]] is [25 99]; /[6 [531 25 99]] is 25; /[12 [531 25 99]] crashes.

(edit: there’s also more at https://urbit.org/docs/tutorials/nock/explanation/.)