The subordinate clause is "When the delivery man arrives" because 1. it is introduced by "when" and 2. we can ask question to the main clause about this one.
Imagery is the use of figurative language to represent objects, actions and thoughts in a way that activates the physical senses of the reader (sight, smell, hearing, tactile and taste). It is what makes the reader involved in a story, by activating feelings he/she might know. It is what makes a story feel realistic.
Its function in literature is to create a graphic representation of a scene, with as much description as possible so the reader can paint a mental picture of what's going on.
So, that through the use of imagery meaning is created could mean that it is what gives a story depth and complexity, it is what makes a story similar to the real world.
