One of the most common misconceptions about cryonics is that patients are simply frozen. Plain freezing is destructive: as water turns to ice, sharp crystals form and tear delicate cell structures apart.
Vitrification is the alternative. By replacing much of the body's water with a cryoprotectant solution and cooling quickly enough, the tissue does not crystallize. Instead it transitions into an amorphous, glass-like solid in which molecules are locked in place without forming ice.
This is the same principle used to preserve embryos, eggs and some organs for transplant research. Vitrification is not perfect, and the cryoprotectants themselves carry some toxicity, but it preserves structure far better than freezing and is the standard of care in modern cryonics. See also how cryopreservation works.