What you're describing is the search for the "elevator speech". How do you oversimplify what you do in a few seconds (the time it takes an elevator to go a short distance), help the other person undertstand a little bit about what you do, and represent yourself and the field in a meaningful way. This is really tough. Here's my stab at it:
"I teach Systems Dynamics and Modeling. Have you ever wondered how families, businesses or other large and complex things work and what's next? We do to. We take complex and changing processes and turn them into models or stories. Then, we use the models or stories to try to understand what's going on and predict what's next? and What makes a difference?"
"Systems Dynamics and Modeling or "story telling" can be learned and used by anyone, and it should be. This is just as important to our country as language and mathematics. Our students start with easy paper and pencil models. Over time we build up to complex models that use specialized symbols and computer software to help us keep track of all the things going on in our story. It's thrilling to use System Dynamics and Modeling to better understand how things work and why."
"We share our models/stories with each other to build them, test them, and make them better. Working with people is one of the most rewarding aspects of this field."
"Systems Dynamics and Modeling is very powerful and even fun."
"Eventually it becomes a whole new way of seeing the world around you."