What does "hh" mean? And what is a "hand"?

hh means hands high

and

a hand is an ancient unit of measurement based on the breadth of the human hand. A horses height is traditionally measured in hands.

A hand is equal to 4 inches or 10.16 centimetres, it is abbreviated to h or when describing the height of a horse hh meaning hands high.

The measurement is taken from ground level to the highest point of the horses withers. The horse needs to be standing square on a smooth level surface. A measuring stick with a spirit level on the cross bar is used. If the horse is shod 1/2 inch (12mm) is deducted off the height.

The above image shows where a horses withers are. Do you know the points of a horse?

Image source: horsevet.co.uk licensed under the Creative Commons Attribution-Share Alike 3.0 Unported