IMAP stands for Internet Message Access Protocol and is a standard protocol for accessing e-mail from a local server. IMAP is a protocol in which e-mail is received and held  on your Internet server. You can view this email using your email client before downloading it. You can create and manipulate multiple folders or mailboxes on the server, delete messages, or search for certain parts within mails.

IMAP is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other one being POP3. Almost all modern e-mail clients and servers support IMAP for transferring e-mail messages from a server, such as those used by Gmail, to a client, such as Mozilla Thunderbird and Microsoft Outlook.

Read more about IMAP at Wikipedia.