IMAP stands for Internet Message Access Protocol, and protocol that allows you to view your email from your server through the email client, webmail client, or mobile device of your choice. This protocol allows you to view your email directly from the server, rather than downloading it to your device. This has the benefit of allowing you to view your email across multiple clients and devices. With the POP protocol, if you downloaded the email to your client, you then wouldn't be able to see it on your phone. However, with the IMAP protocol, you will be able to see the same email content across all of your devices.
For more information on the IMAP protocol check out this helpful knowledge video