Accepted means the campaign was successfully accepted by the recipients' server. Accepted includes all contacts that received the campaign successfully and may or may not have taken other actions.
If a reader also viewed your email, clicked links in your email, or shared your email, you'll also find the contact in your Engaged stats.
Campaigns are tracked using a 1 pixel by 1 pixel image; we call this image a tracking beacon. The tracking beacon is an invisible image in all campaigns that, once downloaded, lets Email Marketing know your contact viewed the email.
Before your reader opens the email and allows images, or clicks a link, we have no way of tracking the recipient behavior. So once your reader downloads the images in your campaign, Email Marketing calls this a "view." If your reader received the campaign successfully, but has not viewed images, or clicked a link, then we call that "accepted."