0m is how you say (decimal)0 because m is the suffix that means decimal.
Other suffixes are f for float, d for double, u for unsigned, and l for long. They can be either upper- or lower-case and u can be combined with l in either order to make a ulong.
Although the suffixes are not case-sensitive, keep in mind what it says in the C# language specification, section 2.4.4.2:
As a matter of style, it is suggested that “L” be used instead of “l” when writing literals of type long, since it is easy to confuse the letter “l” with the digit “1”.