First off, here's what it is: This is a flag that allows you to set a notification on a particular email thread. It's pretty common to turn off notifications for every single email, so this way you can make sure you don't miss an important one. It only appears as an option if you have the cursor in the subject line, so at first I thought my eyes were playing tricks on me. To set it up, just fill in addresses and when you get to the subject, you'll see the bell appear:
Bell shown in subject line on iOS 8 email. Yes that's really my iPhone email signature.
When you see the bell, you can tap on it to see the options for getting notified. It's pretty basic, so your options are limited:
Notification option for email in iOS 8.
Then with it on, send your message. When you get a reply, you'll see it on your lock screen (if you have that enabled). Now you can get notified of just the messages you really need to know about, and find them at a glance in your inbox:
Inbox with a bell shown on a notified thread of email.
To disable notifications once you don't need them any more, swipe right to left on one of the messages in the thread, then tap “More”. At the bottom of the dialog you can tap Disable to stop being notified:
Message options under 'More' in iOS 8 mail.
Now you don't have to miss out on an important message, and just as importantly, you don't have keep being notified once you get the important part.