The easiest way is to use examples from everyday life. You could say something like, when you search for a restaurant near you and click on the first result, someone worked to make that website appear there. That is part of digital marketing. You can also say that the ads you see while scrolling through social media or watching YouTube videos are placed by people in this field. Breaking it down into things people already experience makes it much easier for them to understand without needing technical knowledge. What simple explanation do you think works best for non-digital people?