Flock Mail helps you track when your emails are opened. If you send an email about a proposal to a prospective client or an important email to your colleague, you will get a real time notification when your email is opened. This would help you take action accordingly without speculating or asking if they have read the email or not.
When you compose an email, the 'eye' icon can be toggled to disable/enable open tracking. It is enabled by default for all emails you send.
When someone opens the email, a notification appears on the bell icon. Expand the notification to know about who opened the email and when the email was opened.
If you prefer to selectively enable open tracking for the emails you send, you can toggle the setting by clicking on the Gear icon > Preferences > General > Notification.