It is a pencil gradation system. HB means hard black B means black and 2H means harder.
H (2H, 4H, etc) are the harder leads, and therefore draw a lighter line; and B (2B, 4B, etc) are the softer leads, and therefore draw a darker line. HB is the very middle. The larger the number, the farther away from HB it is. So for instance, 6B will be darker than 2B, and 4H would be lighter than 2H.
Read more: What_do_HB_B_2H_signs_in_pencils_mean
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