ROI embeds a tiny image in your HTML email to track an email open. Each time a recipient opens your ROI email and views the images within it, the tiny image is downloaded from the ROI server. When the image is downloaded, ROI counts that as an event type of Email Open, which is found on the Events Table.
Email Opens are only tracked if the recipients have chosen to show or load images when they view emails. Some email applications block images from displaying by default (like gmail), so unless the recipient clicks the "display images" button, ROI won't be able to track the email open. Utilizing the Email Open in your followup criteria for drip campaigns is not recommended, as the recipients may have their email images turned off.