In literal use, prone and supine indicate contrasting positions of the body: a person lying prone is facing downward while a person lying supine is face up.
Both prone and supine also have meanings that have nothing to do with physical position. Supine, in keeping with the image of one lying comfortably idle, can be applied to those who are willing to be controlled by others, or who show mental or moral slackness, as in "supine obedience" or "supine inaction."
Prone is used in the sense of "having a tendency or inclination," as in "prone to worry" or "accident-prone." This usage is similar to such words as apt, liable, or likely (as in "apt to be late"), but in many instances prone implies a vulnerability to attack or damaging influence, in keeping with the image of one lying face down and unable to see what is approaching.
The word prostrate too has meanings to do with body position. It is used with the very specific meaning of "stretched out with face on the ground in adoration or submission," but is also used simply to mean "lying flat." In figurative use, prostrate means "completely overcome and lacking vitality, will, or power to rise," as in "prostrate in fear."
So while prone, supine, and prostrate have specific meanings with regard to body position, they also come with situational connotations in many cases: prone suggests exposure or vulnerability; supine connotes a position of weakness or passivity; and prostrate implies submission in the face of being overcome.
