There are different Usages of the letter H.
1. the eighth 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 G in a set of items, categories, sizes, etc. e.g. It also denotes the eighth of eight or more hypothetical people or things.
2. Music Chess
In Music it denotes the note B natural in German System. And, in Chess, it denotes the file on the right hand edge of the board, as viewed from White's side of the board.
H = h = hour as in 0900 h means 9 AM. h = hot when c = cold, usually written on taps in Washrooms. GB = GigaByte. H = Hard as in grading of the lead pencils. e.g. "2H Pencil." H = height when D = Depth and W = Width. Besides, H letter of English Alphabet is used in Hungary as International Vehicle Registration. H = Hydrant a sign on the British streets.
H = Hydrogen, the first Element in the Periodic Table, that is a colorless, odourless and highly inflammable gas. In Physics H = Enthalpy which is a thermodynamic quantity which is equivalent to the total heat content of a system, It is equal to the internal energy of the system plus the product of the pressure and volume. Also, H = Henry which is the SI unit of Inductance that equals to an electromotive force of one volt in a closed circuit with a uniform rate of change of current of one ampere per second; named after Joseph Henry an American Physicist who discovered this phenomenon.
5. Interesting Shape Usage
H-Shape is used for making Helipads to facilitate the landing of a helicopter. Interestingly, a large H is drawn directly on the Grounds or Roofs of Skyscrapers and a helicopter lands on it.
The most Common Noun which is used to learn this letter at the Nursery level is Horse; H for Horse.