What is an IMAP sever on Microsoft Outlook?

Answer 1

POP, IMAP or SMTP servers allow you to access other types of email from your outlook account. For example, GMAIL, Yahoo, hotmail... the specific type of server depends on the email service you want to import emails from.

Take a look at : https://support.office.com/en-us/article/pop-imap-and-smtp-settings-for-outlook-com-d088b986-291d-42b8-9564-9c414e2aa040