Sentence:  Thus, in (121) (a) "persuade" is clearly a "three-place <WSD>Predicate</WSD>" — that is, a <WSD>Predicate</WSD> which takes three Arguments: the first of these Arguments is the Subject NP "John", the second is the Primary Object NP "Mary", and the third is the Secondary Object S-bar ["that she should resign"]. By contrast, "believe" in (121) (b) is clearly a "two-place <WSD>Predicate</WSD>" (i.e. a <WSD>Predicate</WSD> which has two Arguments): its first Argument is the Subject NP "John", and its second Argument is the Object S-bar ["that Mary was innocent"]. 
Meaning of the word marked with <WSD> tags:  The part of the sentence (or clause) which states something about the subject or the object of the sentence.