A Man-in-the-Middle (MITM) attack involves spoofing both ends of a communication channel. Suppose A ("Alice") is trying to communicate with B ("Bob"). M ("Mary") gets onto the network between A and B in such as way that A sees M as B, and B sees M as A.
MITM is facilitated by the common store-and-forward architecture of the Internet. In the normal functionality of a network involving routers or switches, packets from A to B are read, processed, and forwarded by various intermediate devices. Likewise, various security devices such as firewalls and proxies insert themselves in the middle of network connection as part of their normal function. So MITM involves a malicious modification of normal networking principles, but normal Internet protocols do little to detect or inhibit such an attack.