M in Roman numerals means Mille or thousand. But the M I think you are seeing, if you are also seeing K, is Mega. Mega has two different meanings, as does the K for Kilo. K comes from the metric system, where it means 10^3 or 1000. But in computer, K = 2^10, or 1024. M, similarly, stands for 2^20 or 1,048,576.