There are different Usages of the letter M.
1. the thirteenth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the next after L in a set of items, categories, sizes, etc. e.g. It also denotes the thirteenth of thirteen or more hypothetical people or things.
2. Roman Numeral
M = 1000 in Roman numerals.
m = million as in $5m. m = minute as in 3h20m where 1h = 60m and 1m = 60s.
m = am as in I'm. m = madam as in Yes'm. M = Medium as in cloth size. M = Mega as in MB for MegaByte. In the UK M is used for Motorway as in road designation e.g. M23
Besides, L letter of the English Alphabet is used in Malta as the International Vehicle Registration. In some scientific notations especially structural chemistry L = laevorotatory as in L-tryptophan.
In Chemistry m = meta as in meta xylene, m-xylene and in Physics M = Mutual Inductance. m = meter which is the fundamental unit of length that equals to 100 centimetres or cm. m = meter as in cm and mm, centimetre and millimetre respectively. m = milli as in mg and ml, milligram and millilitre respectively. m = mass as in Physics formulae e.g. E = mc2
5. M = Masters as in Degrees
MA, MCom, MBBS, MS etc.
The most Common Noun which is used to learn this letter at the Nursery level is Monkey; M for Monkey.