When it comes to email configuration there are two terms (acronyms) that people will be familiar with, POP and IMAP; both are email server protocols which provides users with remote access to their email, however they work quite differently.
POP, or Post Office Protocol, is the least complex of the two, and essentially works by downloading email from an email server to a users computer. When this process has been completed the email is typically removed from the remote server. The major drawback of the POP email protocol is that it is unable to coordinate access/actions across multiple devices. For example, if you check your email from a desktop PC it won’t be apparent which email you’ve read, replied to, archived etc. if you later check your email on a different device.
IMAP, or Internet Message Access Protocol, can coordinate functions across multiple devices and because of this is almost universally preferred. This is made possible because unlike POP, IMAP works by storing email on a remote server which you then access to check your email. Because of this actions are stored on the server side which means it is irrelevant which device you use to access the email as the results will be the same.
With most people in the developed world having access to at least two devices capable of recieving email, it is no wonder that IMAP email services are greatly preferred, hence Gmail, Outlook and Yahoo, huge names in free email services, all use IMAP protocols.