What is IMAP?
IMAP stands for Internet Message Access Protocol. Put simply, it is a method of remotely retrieving your emails from somewhere other than your primary email client on a web browser. For example, instead of accessing your emails from a web browser on your desktop computer, you will be able to access them from your Mobile Device (e.g. iPhone, Android, etc.) or Local Email Client Program (Outlook, Apple Mail, etc.)
What features does IMAP have?
IMAP has what is called two-way communication between your web browser-based email account and your email client (mobile device/email client program). Whatever action you take on your remote device, the same change is instantaneously made in your web-based email account. So if you move an email to the trash or to a specific folder on your remote device, the same action will happen to your primary account.
IMAP also allows you to use multiple devices. Say that you have an iPhone, an Android tablet as well as home and work computers. You'll be able to set up your email account on all of these devices and they will all synchronize with your main account. Any action you take on one of these single devices with then sync with the rest.
You may come across a time when you have no signal or are unable to connect to the internet. In Offline Mode, IMAP allows you to access your messages as you normally would on your remote device. When you finally do have a connection, your remote device will then sync with your web-based account.
Finally, IMAP prevents downloading the same message twice or skipping over messages. This is due to the two-way communication of your remote device(s) and your web-based email account.