I feel like a dumbass

I totally read your comment as “Megahertz or Millihertz”

Now I realize it should just be Meters•Herts or m•Hz

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

By convention, compound SI units (e.g. N•m) are separated by a space or multiplication dot and are not just smashed together. The lack of such a separator in the above example implies the “milli” prefix.