Standing alone ‘m’ is metre.

With a decimal modifier ahead of it, ‘m’ is metre

Immediately before a symbol it is milli (one thousandth)

mm - the first m is before the metre symbol, it is the milli multiplier, the second m is after a multiplier, so it is the symbol for metre

mHz - the m is before the symbol Hz (for Hertz) so it is the milli multiplier

km - the m is after the K (kilo, thousand times) multiplier, so it is metre

You probably spell metre ‘meter’, but a meter is a whole different word in English