IMAP is a protocol many applications use to retrieve mail from your mail server
Internet Message Access Protocol (IMAP) is a communication protocol used by applications and servers for retrieving emails from email servers. Email programs like Microsoft's Outlook and Mozilla's Thunderbird use this protocol to contact servers and download messages for local storage and viewing offline. It is widely used by most email service providers, including Gmail and Yahoo!.
For more information, read the Wikipedia article. For technical information read the RFC 3501.