In short, DIN is a standardized and calibrated rating of how “stiff” the release of a binding is set to. In other words, it tells you how much forward falling force and twisting force will release you from your bindings. Several factors like weight, height, skiing ability, age, and boot-sole length will come into play when determining your ideal DIN.
While it’s best to get your bindings checked and adjusted by a professional ski technician, there are online DIN calculators to help you get started.