what is this mentioned full form all about?
It  is a fictional character in Ian Fleming's James Bond book and film series; the character is the Head of the Secret Intelligence Service.
M (James Bond)
 is a fictional character in Ian Fleming's James Bond book and film series; the character is the Head of the Secret Intelligence Service
what does m stand for in james bond films MI6
wiki:19844643
M (James Bond) [SEP] Introduction
M is a fictional character in Ian Fleming's James Bond book and film series; the character is the Head of the Secret Intelligence Service—also known as MI6—and is Bond's superior. Fleming based the character on a number of people he knew who commanded sections of British intelligence. M has appeared in the novels by Fleming and seven continuation authors, as well as appearing in twenty-four films. In the Eon Productions series of films, M has been portrayed by four actors: Bernard Lee, Robert Brown, Judi Dench and Ralph Fiennes, the incumbent; in the two independent productions, M was played by John Huston, David Niven and Edward Fox.