MIME stands for Multipurpose Internet Mail Extensions, and was originally designed for sending multimedia electronic mail. The two main things it does are specify in a standard way what type of media the contents of a message actually are, and what form they've been encoded in for transmission.
When Tim Berners-Lee was originally designing what would go on to become the World Wide Web, he had exactly this same requirement - he needed a server to be able to specify to a client what a response contained and how it had been encoded. The email people (Nathaniel Borenstein and Ned Freed) had got there first, and had already specified MIME, so there was no need to re-invent the wheel, even though for this purpose the name is a little odd.