"Her equitation sucked."
"Gaming always makes my equitation go downhill."
Equitation is a somewhat fancy term for the actual skill of a rider in handling the horse, keeping correct form, etc. At a show, equitation classes are judged entirely on the ability of the rider with the horse, in theory, not being taken into account. (Equitation horses are chosen to be quiet, not particularly flashy, and easy to ride).
In most cases, if a horseman is criticizing somebody else's equitation they refer to such things as the posture in the saddle, position of the hands, etc - you can still ride effectively but not have the best equitation.