Instructional design is often a field that great employees and facilitators find themselves in by accident. A common assumption is that because someone is good at their job they will naturally be good a developing the training. This is not always the case. Instructional design is a learning discipline that requires intentional thought and development.
This article does a great job of introducing you to the overall concept of what an instructional designer does. It also touches on the use of the ADDIE process which is the bedrock of the instructional design process.
An instructional designer applies this systematic methodology (rooted in instructional theories and models) to design and develop content, experiences, and other solutions to support the acquisition of new knowledge or skills.