For square units, the prefix is applied to the base unit - that is, before squaring it, not after. Thus, a square kilometer (km2) = 1000 m times 1000 m, which is equal to a million square meters.
MM is an abbreviation that may represent one million (See 1st related link below). M typically represents one thousand, so "MM" literally conveys "a thousand thousand". --M is the roman numeral for...