In relation to credit ratings, 'm' is a suffix added to the credit rating to indicate that it applies to a money market fund (rather than to any other type of entity).
So for example "AAAm" would indicate a rating of AAA in relation to a money market fund.
'm' also means one million (1,000,000).
For example EUR 100m = EUR 100,000,000.
Sometimes written "M", "mn' or 'mio'.