It was the access method for the initial version of Time Sharing Option (TSO).
The MCP handles communications with the terminals, identifies input messages and starts application programs to process them as required.
This is similar in concept to the much later internet service daemon (inetd) in unix and other systems.
The MCP is assembled by the user installation from a set of macros supplied by IBM.
These macros define the lines and terminals comprising the system, the datasets required, and the procedures used to process received and transmitted messages.