In a sentence talking about people, the people are subjects of that sentence. Subjects are basically what is being talked about.
Because there are two people being talked about, Bob and Al, the subjects are counted as one, or compounded. This just means that you read the sentence as [Bob and Al] instead of [Bob] and Al.
Compound verbs follow the same concept, but for action words. For example, “to sing and to dance”. However, in this case since the boxed words are subjects, they are a compound subject.
