m means mille, Latin for thousand. mille mille is therefore 1 million.
It's used in other business contexts too. If you've ever paid for online ads, you'll know that CPM means cost per mille, the price of 1000 ad impressions.
I think it's meant to be Roman numeral abbreviation for a thousand-thousand. You would think MM would be two thousand following the convention, but it's multiplicative like 10^6.
30 million. don't know why, but it's probably to do with "m" already being used elsewhere, or something.
M is 1000 in Roman numerals, that is the origin in accounting. It should be upper case though.
In finance they use it because "M" is the roman numeral for 1000. MM is 1000*1000.