In terms of dollars, MM stands for 1,000,000. This is based off the Roman numeral "M," which stands for 1,000, and MM, which is used to indicate 1,000,000.
There are actually a few different ways that 1,000,000 can be expressed when it comes to writing dollar amounts. One example would be having it listed as $4,000,000. Another person may use $4MM to indicate 4 million. Million can also be represented using "mn" and "m," so an individual may see $4m, $4mn or simply $4 million. Another option is to use the "M" for 1,000 and represent $4 million as $4,000M.