Sometimes a subject may be more than one thing or person. They have more than one noun or pronoun and are called compound subjects.
Here, there are three main subjects who are Debra, Harry, and I. This is a compound subject.
Here, there are three things that covered Anne's diary; yellow stickers, old photographs, and glitter. This is also a compound subject.
Sometimes one subject may be doing more than one thing. There may be more than one verb and these are called compound predicates.
Betty is the one subject, but she is doing two things; walking and admiring. So, "walks along the ocean" and "admires the waves" is a compound predicate.
Here, Mary is one subject who is doing two things; singing and whistling. So, "sings and whistles to the song" is a compound predicate.