Essentially "cryo treatment" refers to a portion of the "heat treat" process. As it applies to knife blades, once the steel is hardened, and then given an initial tempering, the blade(s) are placed in an extremely cold environment. This most common is liquid nitrogen (-300F). This process converts retained austenite to martensite. By transforming retained austenite, a couple points of Rc hardness can be realized, which is addressed in further heat treating steps.
Some folks will use dry ice and acetone in an attempt to cryo, but the max you can achieve with that combo is approx -108F. This MAY have some benefits on simpler steels, but is not cold enough for more complex/high alloy steels.
Personally, I cryo treat my blades in liquid nitrogen. On some of the stainless steels such as 13C26, ATS-34, and 154CM it make a night and day difference in the cutting performance of the blade(s). My setup consists of a "dewar" tank (used to preserve semen for artificial insemination by Ranchers/Veterinarians) that holds up to 56lbs of liquid nitrogen (28 liters). After the blades go through their first tempering cycle, I place them on a stainless wire, and lower them onto the tank, where they remain for 8-12 hours. I then take the blades out, and place them between two layers of Kawool, which allows them to warm slowly to room temp. Once they are at room temp, they go back into the tempering oven for 2 hours, at 25F higher than the previous temper(s).
I've had great results "cry treating" 52100, 5160, CruForge V, as well as the steels I mentioned above. Once you get over the sticker shock of a dewar tank, and the fact that you'll have to fill the tank nearly every time you use it, it's just another step we can take to produce a better blade.
NOTE: Something I did, that makes my tank of liquid nitrogen last: I built a plywood box, (including a hinged lid) lined with 2" thick rigid foam insulation. As soon as I get the tank home from a fill-up, I place it in the box, and fill all the surrounding air space with vermiculite, then place a layer of fiberglass insulation over the top of the tank...then shut the lid. With the dewar full, and in the box, it will routinely last 2 months, give or take for weather conditions.