HRC is short for Hardness Rockwell C Scale. There are different Rockwell hardness scales, but the C scale is the industry standard for knives.
The higher the HRC means the harder the steel which gives a longer-lasting edge and more acute cutting angles are possible. On the flip side too high an HRC and the blade becomes brittle.
Generally, our knives are in the range of 60-61. This degree of hardness enables our Japanese knives to be thin, hard, and precise without being overly brittle. However, we also have some for more robust work in the 56-58 range.