ROI means return on investment, which basically asks whether the money spent on marketing brought back more money than it cost. The best way to explain this to a client is to keep things simple. Instead of showing them charts full of numbers, show them how many new customers came in and what they spent. You can also compare the period before the campaign to the period after. Clients respond better to money numbers than to marketing terms. What format do you find works best when presenting results to non-marketing clients?