What does m stand for in math like length?

'm' ( small/lower case), means 'metre'.

Careful with the spelling;-0

Metre; European spelling

Meter ; American spelling.

The Word ' metre' has its root in Latin, and means 'measure'.

Here is the Metric Table. of length

10 mm = 1 cm

100 cm = 1 m

1000 m = 1 km.