Perhaps the best preliminary description would be a comparison with software engineering. That discipline consists of the design, development, and implementation of abstract data structures and processes to be used in computers. Though its process deals with abstract structures, its practitioners are engineers, in the same way, that the people who design large-scale electrical frameworks are.
In a similar fashion, systems engineers deal more with abstract structures than with concrete things, though even this is not always true. How, then, do we describe the role they play? Defining “Systems” Perhaps the best way to understand this field is to break down the title word for word.