Modeling, of any kind, involves creating simplified representations of reality. Modeling is an important tool in science and many other disciplines as it allows the modeler to test alternative scenarios or hypotheses.
Systems are parts of the real world.
They are made up of interrelated things and the relationships among those things. This definition is vague because systems are very diverse, however thay all have certain critical elements in common. Dynamic systems are those that change over time. Most interesting systems are dynamic. Putting it all together, dynamic systems modeling is the creation of a representation of a group of things and their interrelationships in order to study the changes in those things and their relationships over time.