What do the initials "HB" on a pencil stand for?

The degree of hardness of a pencil is printed on the pencil. The degrees of hardness of pencils are roughly divided into four groups: B stands for "black". These pencils are soft. H stands for "hard". HB stands for "hard black", which means "medium hard". F stands for "firm". Sometimes the alphabetic characters are preceded by numbers. They make an even more precise statement about the degree of hardness or color intensity of the pencil. The higher the number, the harder, softer, or blacker the pencil is.

Source: Eberhard Faber