Distribution: "We distinguish a distributed term (subject or predicate) from an undistributed term in this manner: A distributed term is one modified by 'All' or 'No.' When a term is modified by "some," it is undistributed," because it does not apply to all cases.
"There are only four standard form propositions. Each consists of a subject and a predicate. In each form, the subject and the predicate are joined together by is or are, the copula. The relation between the subject and predicate is identified by the use of: All, No, Some, or Some ... not.... If a and b stand for the subject and predicate terms, respectively, the four forms are: (1) All a is b, (2) No a is b, (3) Some a is b, and (4) Some a is not b."
Now, to understand why the "predicate term" is undistributed, we have to know what a "predicate" is.
One dictionary says,
"7. Grammar: belonging to the predicate: a predicate noun.
8. Grammar: (in many languages, as English) a syntactic unit that functions as one of the two main constituents of a simple sentence, the other being the subject, and that consists of a verb, which in English may agree with the subject in number, and of all the words governed by the verb or modifying it, the whole often expressing the action performed by or the state attributed to the subject, as is here in Larry is here."
The key to identifying the "subject term" and the "predicate term" is the verb like "are/is" between them. The inclusion/exclusion words, like "some" or "all", tell you the relationship between the subject term and the predicate term.