In a simple explanation we design instructional material.
Instruction design originated in the military. However, the position can be found in a multitude of different fields.
Aligning learning objectives to learning activities and learning. It is common sense for good teachers. What do I want my students to learn?
It is the entire process of analysis of learning needs and goals and the development of a delivery system to meet those needs. It includes development of instructional materials and activities; and tryout and evaluation of all instruction and learner activities. (Penn State)