From my knowledge a non-recursive function would be easier than a recursive one, since a non-recursive one will not call itself while a recursive one can. But the quoted phrase seems to imply it must be something more difficult instead...
Problem is, wikipedia's page https://en.wikipedia.org/wiki/Non-recursive_function does not define it either, it just redirects you to the recursion page (though not recursively, so wikipedia did not go for the joke) which does not define it either.
A “non-recursive function” would be more powerful than that; or, conversely, it would be a function which cannot be computed using a Turing machine. Alternatively, these are also know as super-recursive functions [2].
[1] https://en.wikipedia.org/wiki/Computable_function [2] https://en.wikipedia.org/wiki/Super-recursive_algorithm
"the [Church-Turing] thesis has several possible meanings:
1) The universe is equivalent to a Turing machine; thus, computing non-recursive functions is physically impossible. This has been termed the Strong Church–Turing thesis, or Church–Turing–Deutsch principle, and is a foundation of digital physics."