New functions are defined using lambda. This takes two arguments returning a new closure. The first argument is the new function's arguments the second argument is the body of the new function [lambda args body]. A complete example is [lambda '[x:1] '[* x x]].
Notice again how both arguments are data. The first argument is a map that defines the argument names. The argument map defines a mapping of new names used in the function body to names taken from when the function was called.