[tizoc]

It means that to be portable you have to wrap symbols that represent functions in (function <the-symbol>) when passing them as arguments. Not doing so will work on some ports but not on others, and should be considered "undefined behaviour".

Portable:

(foldl (function +) 0 [1 2 3])

May work depending on the backend:

(foldl + 0 [1 2 3])