Good question! 

When I started my new role I had no idea either. I knew what I had to do and I thought I could do it well but what exactly is an Instructional Designer. I found this…

“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. Instructional designers ought to begin by conducting a needs assessment to determine the needs of the learning event, including: what the learner should know and be able to do as a result of the training or learning solution, and what the learners already know and can do.

Instructional designers are then responsible for creating the course design and developing all instructional materials, including presentation materials, participant guides, handouts, and job aids or other materials. Instructional designers are commonly also responsible for evaluating training, including assessing what was learned and whether the learning solution led to measurable behaviour change.”

 ^Not sure where I found this text.