Those abbreviations indicate the relative hardness of the knife's steel, as measured on the "Rockwell scale." And while we could spend a lot of time drilling into the metallurgical details of Rockwell hardness, we'll try to keep this explanation simple and useful.
A lower Rockwell number means that the steel is softer, relatively speaking, while a higher number indicates a harder steel. Sometimes you'll see Rockwell hardness expressed as a single value, like "59 RC," and other times as a range, like "58-60 HRC."
A steel's final hardness is established by the knifemaker during the heat-treating process. Different steels are known to perform best within different Rockwell ranges.
Generally, relatively soft knife steels (lower Rockwell numbers) are easier to sharpen but don't hold an edge as long, and a harder steel (higher Rockwell) is more resistant to wear, taking more work to achieve an edge but holding it longer.
Too soft and a knife is basically useless, rolling its edge easily or requiring frequent sharpening. Too hard and the steel becomes brittle and prone to chipping or breakage.
Now when you see those "RC" or "HRC" ratings, you should have an idea of what you're getting. One question remains, however.
Is Rockwell hardness important?
For the knifemaker, yes -- getting the hardness "right" is a crucial part of crafting a knife that performs as designed. Still, it's only one piece of the puzzle. Other factors -- the type of steel, blade geometry and the knife's intended purpose, to name just a few -- are important, too.
If knifemaking is a skill, then heat-treating is truly an art form, practiced at the highest level by relatively few. We're proud to count the companies and makers represented on KnivesShipFree among that elite group.
So for the rest of us -- those of us who aren't knifemakers or metallurgists, that is -- it really is enough to grasp just the basics of Rockwell hardness. Rather than choosing our tools "by the numbers," we should interpret what those numbers mean and gravitate toward makers who do it right and knives that suit our purposes.