What is the rank operator?

The concepts of leading axis and item are generalized by treating an array as a frame defined by the array’s leading m axes holding cells of rank n defined by the array’s trailing n axes, where m+n is the rank of the array. A function f is applied to all cells of rank n with the expression f@n. The rank operator (@) applies uniformly to all functions of one or two arguments: primitive, derived, or defined, except Assignment and (because of its syntax) Bracket Indexing (@ does apply to Choose, which is semantically equivalent).