The short answer is that IMAP connects to your email providers' server and is used for receiving email while SMTP is used for sending mail to other servers.

IMAP (Internet Message Access Protocol) is a protocol that requests that your email provider will save all of your messages and folders onto its own servers. This way, whether you are using an email client or webmail, you will be able to connect to your email account and view anything you have received because it's always stored on their server.

STMP (Simple Mail Transfer Protocol) is the protocol for sending emails out of your account to other destinations. It enables your email client to connect to the server of your provider to send outgoing mail, especially to other servers.