While impressions represent how many people laid eyes on your content online. Customer engagement represents how interesting your customers find your content. For example, how many downloaded a PDF version of your content, how many email opt-ins? How many clicks did you received? How many viewed a video? How many people were expose to and liked your content on social media - Facebook, Twitter, Google+, LinkedIn etc? How many phone calls, leads, proposals, ROI etc? All of these factors tell you if your target audience were engaged or not.