ROI means Return on Investment. In SEO at starting you spend money (investing) on site for promotion and after some period user visit your site, buy the product.
ROI means "Return On Investment". with ROI analysis you can have an idea of what you have spent & how much you have gained. its a investment vs profit analysis.
Full form of ROI is Return On Investment. It's measure how much gain profit from that marketing strategy.
ROI - Return on Investment, means when you are starting SEO for your website, you are investing some money for promotion of your website using adwords or PPC or anything else and you get good traffic of visitors which converts in buyer or customer of your website. In simple words, ROI means something that you will get back on your investment.
In internet marketing ROI stands for Return On Investment that simply means all the results of your investment which you spends for promotion of your web site and business.