Pulse is the bell at the top of the desktop app. It collects updates that aren't new mail: opens of emails you sent, scheduled sends, shared conversations, comments, mentions and assignments.
Steps
Click the bell at the top right, just left of your profile picture, or press N. A red dot means there's something new.
Click an item to open its email or conversation. For a scheduled email that failed, go to Sent › Scheduled instead.
To remove an item, click × on it. To empty the list, click Clear all.
You'll see an item when:
An email you scheduled was sent (Email sent) or couldn't be sent (Failed to send email).
Someone opened an email you sent, if you asked to be notified about opens.
Someone shared a conversation with you, commented on one, mentioned you or assigned you a conversation.
Good to know
Pulse is on desktop only. On your phone, opens, mentions and assignments arrive as notifications instead.
Opening Pulse marks everything in it as read. Read and dismissed items update on your other computers too.
Emails you send right away don't appear in Pulse; only scheduled ones do.
New items can also show up as a desktop notification.