When a user leaves the network with which his device is associated (home network) and enters the domain of a foreign network, the foreign network uses the Mobile IP protocol to inform the home network of a care-of address to which all packets for the user's device should be sent.
A common analogy to explain Mobile IP is when someone moves his residence from one location to another. Person moves from Boston to New York. Person drops off new mailing address to New York post office. New York post office notifies Boston post office of new mailing address. When Boston post office receives mail for person it knows to forward mail to person's New York address.