The document defines and provides examples of what constitutes a subject in a sentence. A subject is who or what the sentence is about and differs from a predicate, which is what the sentence is, does or feels. Examples given are that in the sentences "The daisy danced in the wind" and "The frog jumped from one leaf to another", the subjects are "Daisy" and "frog" respectively, as they are the topics being described by the sentences.