Claims that “HB” stands for “hard black” are mistaken. The French played a major rôle in the history of the pencil, and the standard system for designating the quality of pencil lead was introduced by a Frenchman. Pencils in the ‘H’ (“haut”) range have a high amount of binder relative to graphite, so they make light lines. Pencils in the ‘B’ (“bas”) range have a low amount of binder relative to graphite, so they make dark lines. The F and HB are in-between the H and B ranges, with the F having a bit more binder than the HB. (The ‘F’ stands for “fine” because it will hold a finer point than an HB.)
An American Nº 2 will be close or equivalent to an HB.