Both MCM and kcmil mean 1,000 circular mils, which is a unit used to measure the area of the circular cross section of a wire. The terms MCM and kcmil are used interchangeably by the National Electric Code to describe wires that are larger than 0000 American wire gauge.
One mil is equal to 1/1000 of an inch, and one circular mil is the area of a circle that has a diameter of one mil. The reason MCM and kcmil both mean 1,000 circular mils, is that the letter M in roman numerals is equal to 1,000, so MCM is 1000CM, or 1,000 circular mils. Similarly, the prefix kilo means 1,000, so kcmil is equal to 1000cmil, or 1,000 circular mils.