Can you explain this experiment on grammar acquisition to me?

Hey everyone, since English is not my first language and I am also new to evolutional linguistics, I do not understand the description of this experiment so well.

Fitch and Hauser (117) recently completed a study comparing finite-state and phrase-structure grammar acquisition in human adults and tamarins. The phrase-structure rule tested was AnBn, where A and B were each represented by one of a set of eight different CVs. The rule therefore specified both a set of consistent strings (n A's must precede n B's) and a set of inconsistent strings; the latter consisted of violations of order (B tokens precede A tokens) or of patterning (alternations of A's and B's such as ABAB). Results showed that human adults rapidly learned this rule implicitly, distinguishing consistent and inconsistent strings. Tamarins, in contrast, failed in three separate experiments testing their ability to acquire this grammar, but they readily mastered a finite-state variant (ABn) implemented with the same stimuli and testing conditions. This suggests that tamarins have a limited capacity to learn the type of long-distance hierarchical dependencies necessary to achieve the class of phrase-structure grammars. If true, this limitation would place severe restrictions on their capacity to learn any natural human language.

So where did the tamarins exactly fail? What did they not succeed to do? I doubt they could form sentences with finite grammar. Was it about understanding?

I'd love some help here, thank you! :)