It means it's a decimal literal, as others have said. However, the origins are probably not those suggested elsewhere in this answer. From the C# Annotated Standard (the ECMA version, not the MS version):
decimalsuffix is M/m since D/d was already taken by
double. Although it has been suggested that M stands for money, Peter Golde recalls that M was chosen simply as the next best letter in
A similar annotation mentions that early versions of C# included "Y" and "S" for
short literals respectively. They were dropped on the grounds of not being useful very often.