things like someone's physical or mental fitness to handle the demands of being the president of the United States.
Hillary certainly seems to have some significant mental/physical issues that would adversely impact her ability to carry out the necessary functions of the office.
https://www.youtube.com/watch?v=Zr1IDQ2V1eM
https://www.youtube.com/watch?v=i-AyQHKlWr8
http://www.inquisitr.com/3397824/hillary-clinton-health-fears-spark-concern-is-candidate-healthy-enough-for-presidency/