Internet Message Access Protocol (IMAP) is a standard email protocol, that allows users to access emails or messages stored on a remote mail server from virtually any local mail client and device that supports the IMAP protocol.
IMAP can be thought of as a remote file server accessible by email clients. Unlike with POP3, the mails will retain on the mail server. Multiple client applications on different devices simultaneously can access the messages and see whether a certain message has been read, flagged for urgency, replied, and so on.
For example, you can read your mails on your works' desktop computer, laptop computers or your smartphone, using email clients as Thunderbird, Microsoft Outlook, a webmailer, or any other mail app of your choice.