I had to google "unfree data types". This seems to refer to data types that are unstructured and therefore no one thought of a way to represent in syntax. For instance we can ask if the first item of a list (a structured data type) is equal to the last, but we do not have good syntax (nor semantics) to ask what are the pairs that occur in a collection. That's why we keep having to translate between those structures in order to do some operations or ask certain questions.

Very cool stuff.