Tern tracks when recipients open your emails and click links, on emails you send from Tern through your connected email inbox. You don't need to turn anything on.
What the icons mean
Sent messages show a status icon:
Three dots: no engagement yet
Single checkmark: the recipient opened your email
Double checkmark: the recipient clicked a link
Click the icon to see how many times the email was opened, which links were clicked, and when.
When you send separately to several people, each person gets their own email, so opens and clicks are counted per recipient. "2 opens" on one traveler's email means that person opened it twice.
Why an email shows no tracking icon
Your inbox isn't connected, or it's connected for calendar only. Emails then go from [email protected], and those emails aren't tracked. Connect your inbox with both email and calendar permissions under Settings, then Email & Scheduling.
The email was sent before your inbox was connected. Tracking only applies to emails sent through the connected inbox.
Click tracking covers standard web links (https). An open can go unrecorded when the recipient's email app blocks images.