There's an old convention of M = mille = thousand, and the modern convention of M = million. To avoid confusion, MM is sometimes used to be very clear it means

There's an old convention of M = mille = thousand, and the modern convention of M = million. To avoid confusion, MM is sometimes used to be very clear it means million, especially in accountancy and legal circles. This has arguably outlived anyone seriously using M=mille, but the practice carries on