Sheath cleaning is a hygienic process occasionally needed by male horses, both geldings and stallions, wherein a caretaker, groom or veterinarian checks the horse's "sheath," the pocket of skin that protects the penis of the horse when it is not in use for urination (or, in the case of stallions, breeding).
Even when Tsar Alexander I asked Orlov to sell him several stallions, Orlov only agreed to sell geldings.